Replace root redirect with spec listing index page and build issues lists (#13433)

Instead of redirecting to drafts.csswg.org, build-index.py now generates
an index page with search, sortable views, and per-spec resource links.

Features:
- Search/filter bar for finding specs instantly
- Recent (by date) and Grouped (by shortname family) sort modes
- Activity indicators and relative timestamps
- Per-spec links to GitHub issues, PRs, commit history
- Links to Disposition of Comments documents where they exist

Also adds a workflow step that runs bikeshed issues-list on all .bsi and
issues-*.txt source files, regenerating the Disposition of Comments HTML
pages during deployment.

Relates to #12054

Author: sideshowbarker

.github/workflows/build-specs.yml

@@ -57,6 +57,12 @@ jobs:
             SHORT_DATE="$(date --date=@"$TIMESTAMP" --utc +%F)"
             bikeshed -f spec "$file" "${file%Overview.bs}index.html" --md-date="$SHORT_DATE" --md-Text-Macro="BUILTBYGITHUBCI foo"
           done
+      - name: Build issues lists
+        run: |
+          for file in ./**/*.bsi ./**/issues-*.txt; do
+            echo "  $file"
+            bikeshed issues-list "$file" || true
+          done
       - name: Build index & symlinks
         run: python ./bin/build-index.py
       - run: rm -rf ./.git{,attributes,ignore}