Add a script to check documentation for ToC entries. (#8356)

This script verifies that each document in the docs and architecture directory
has a corresponding table-of-contents entry in its README file.  It can be run
manually from the command line.

- Hook up this script to run in CI (optional workflow).
- Update ADR ToC to include missing entries this script found.

.github/workflows/docs-toc.yml

@@ -0,0 +1,20 @@
+# Verify that important design docs have ToC entries.
+name: Check documentation ToC
+on:
+  pull_request:
+    push:
+      branches:
+        - master
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: technote-space/get-diff-action@v6.0.1
+        with:
+          PATTERNS: |
+            docs/architecture/**
+            docs/rfc/**
+      - run: ./docs/presubmit.sh
+        if: env.GIT_DIFF