docs: add a few redirects

Follow-up to #4215, in which we reorganized the guide for readability.
Some of the existing content got shuffled into subroutes, and already
one person in Discord has pointed out that we broken URLs. Tacking on
some http redirects via mdbook [0].

[0] https://rust-lang.github.io/mdBook/format/configuration/renderers.html#outputhtmlredirect

docs/guide/book.toml

@@ -27,3 +27,17 @@ warning-policy = "error"
 # The buf.build website doesn't support HTTP HEAD calls, returning 405.
 # Only relevant when `follow-web-links=true`.
 exclude = ['buf\.build']
+
+[output.html.redirect]
+# Renames in GH4215.
+"pd.html" = "node/pd.html"
+"pd/chain-upgrade.html" = "node/pd/chain-upgrade.html"
+"pd/debugging.html" = "node/pd/debugging.html"
+"pd/install.html" = "node/pd/install.html"
+"pd/join-testnet.html" = "node/pd/join-testnet.html"
+"pd/requirements.html" = "node/pd/requirements.html"
+"pd/validator.html" = "node/pd/validator.html"
+"pclientd.html" = "node/pclientd.html"
+"pclientd/build_transaction.html" = "node/pclientd/build_transaction.html"
+"pclientd/configure.html" = "node/pclientd/configure.html"
+"pclientd/rpc.html" = "node/pclientd/rpc.html"