Fix blank top-level 404 page (#84)

Fixes #7 in an
unfortunately clever, yet still appropriate way.

See the comment in `src/js/docsPluginWithTopLevel404.js` for greater
detail.

Tests:
- http://localhost:3000 -> ✔️ 
- http://localhost:3000/ -> (redirects to http://localhost:3000)
- http://localhost:3000/2.20/docs/introduction/welcome-to-pants ->
✔️
- http://localhost:3000/2.20/nope -> (Page Not Found) ✔️
- http://localhost:3000/nope -> (Page Not Found) ✔️ 
- http://localhost:3000/sponsorship -> ✔️

docusaurus.config.js

@@ -68,47 +68,7 @@ const config = {
       "@docusaurus/preset-classic",
       {
         debug: process.env.NODE_ENV !== "production",
-        docs: {
-          sidebarPath: require.resolve("./sidebars.js"),
-          routeBasePath: "/",
-          disableVersioning,
-          onlyIncludeVersions,
-          lastVersion: onlyIncludeVersions ? undefined : versions[1],
-          versions: {
-            current: {
-              label: `${currentVersion} (dev)`,
-              path: currentVersion,
-            },
-            ...(disableVersioning
-              ? {}
-              : versions.reduce((acc, version, index) => {
-                  acc[version] = {
-                    label:
-                      index === 0
-                        ? `${version} (prerelease)`
-                        : index < 3
-                          ? version
-                          : `${version} (deprecated)`,
-                    banner:
-                      index == 0
-                        ? "unreleased"
-                        : index < 3
-                          ? "none"
-                          : "unmaintained",
-                    noIndex: index >= 3,
-                    path: version,
-                  };
-                  return acc;
-                }, {})),
-          },
-          remarkPlugins: [captionedCode],
-          editUrl: ({ docPath }) => {
-            if (docPath.startsWith("reference/")) {
-              return undefined;
-            }
-            return `https://github.com/pantsbuild/pants/edit/main/docs/${docPath}`;
-          },
-        },
+        docs: false, // NB: See `docsPluginWithTopLevel404.js` reference below
         blog: includeBlog && {
           showReadingTime: true,
           editUrl: "https://github.com/pantsbuild/pantsbuild.org/edit/main/",
@@ -304,6 +264,50 @@ const config = {
     },
   },
   plugins: [
+    [
+      "./src/js/docsPluginWithTopLevel404.js",
+      {
+        sidebarPath: require.resolve("./sidebars.js"),
+        routeBasePath: "/",
+        disableVersioning,
+        onlyIncludeVersions,
+        lastVersion: onlyIncludeVersions ? undefined : versions[1],
+        versions: {
+          current: {
+            label: `${currentVersion} (dev)`,
+            path: currentVersion,
+          },
+          ...(disableVersioning
+            ? {}
+            : versions.reduce((acc, version, index) => {
+                acc[version] = {
+                  label:
+                    index === 0
+                      ? `${version} (prerelease)`
+                      : index < 3
+                        ? version
+                        : `${version} (deprecated)`,
+                  banner:
+                    index == 0
+                      ? "unreleased"
+                      : index < 3
+                        ? "none"
+                        : "unmaintained",
+                  noIndex: index >= 3,
+                  path: version,
+                };
+                return acc;
+              }, {})),
+        },
+        remarkPlugins: [captionedCode],
+        editUrl: ({ docPath }) => {
+          if (docPath.startsWith("reference/")) {
+            return undefined;
+          }
+          return `https://github.com/pantsbuild/pants/edit/main/docs/${docPath}`;
+        },
+      },
+    ],
     [
       "@docusaurus/plugin-client-redirects",
       {

src/js/docsPluginWithTopLevel404.js

@@ -0,0 +1,56 @@
+import { default as pluginContentDocs } from "@docusaurus/plugin-content-docs";
+
+/*
+You: WTH is going on here?!
+Me: Ok, umm, please let me explain...
+  So, there's this bug: https://github.com/facebook/docusaurus/issues/9688. Basically, because we
+  use `routeBasePath` set to `/`, the docs plugin hogs all `/*` routes that aren't hogged by another
+  plugin. But that means that "bad" top-level URLs like `/foo` render empty.
+You: (nodding)
+Me: It's complicated, you can see the comments from the maintainers. Something about `react-router-config`.
+  Anyways, there was a suggestion about how to fix it by "automatically add[ing] a `*` not found route
+  to all subroutes". This emulates that behavior.
+You: I think I get it, but how does this work, exactly?
+Me: Ah yeah. So firstly, we use this plugin instead of the docs plugin. This plugin is basically a
+  re-export of the docs plugin, but with the `addRoute` plugin API monkeypatched. The monkeypatching
+  adds a `*` not found route as a subroute.
+You: I see. Well, OK I guess. Carry on.
+*/
+export default async function patchedPluginContentDocs(context, options) {
+  const result = await pluginContentDocs(context, options);
+  const actualContentLoaded = result.contentLoaded;
+  result.contentLoaded = async function ({ content, actions }) {
+    let docsRouteConfig = undefined;
+    const actualAddRoute = actions.addRoute;
+    actions.addRoute = function (routeConfig) {
+      if (docsRouteConfig !== undefined) {
+        throw "Expected only one call to addRoute!";
+      }
+      docsRouteConfig = routeConfig;
+    };
+    const result = await actualContentLoaded({ content, actions });
+    docsRouteConfig.routes.push({
+      path: "/*",
+      component: "@theme/NotFound/Content",
+      /* NB: Routes are sorted based on a heuristic: https://github.com/facebook/docusaurus/blob/d94adf6a6c925f2bd5ef09a75c5e8a66a0b7e7f9/packages/docusaurus/src/server/plugins/routeConfig.ts#L30
+          Our route must come last based on this heuristic. In order to accomplish that:
+            - We must have at least one route, or else we unconditionally come first.
+              It'll never get matched, but still must be valid
+            - Our priority must be _lower_ (with the default of 0).
+      */
+      priority: -1,
+      routes: [
+        {
+          path: "/*",
+          component: "@theme/NotFound/Content",
+        },
+      ],
+    });
+
+    actualAddRoute(docsRouteConfig);
+    return result;
+  };
+  return result;
+}
+
+export { validateOptions } from "@docusaurus/plugin-content-docs";