ci: don't run long tests on docs-only PRs

I want to keep the docs up to date, and sometimes that requires fussy
management of version numbers, particularly as we invest in multi-branch
workflows to support point releases (e.g. #4545). If *only* the docs
changed, skip the long-running rust tests, because we don't need to
validate rust functionality on docs changes.

.github/workflows/rust.yml

@@ -1,5 +1,9 @@
 name: Rust CI
-on: pull_request
+on:
+  pull_request:
+    # Don't run Rust tests if only docs changed.
+    paths-ignore:
+      - 'docs/**'
 
 jobs:
   test:

.github/workflows/smoke.yml

@@ -1,9 +1,9 @@
 name: Smoke Test
 on:
   pull_request:
-  push:
-    branches:
-      - main
+    # Don't run Rust tests if only docs changed.
+    paths-ignore:
+      - 'docs/**'
 
 jobs:
   smoke_test: