CLDR-16526 spec: link check spec/rfc

- run lychee action

.github/workflows/gh-pages.yml

@@ -59,6 +59,16 @@ jobs:
       - name: Lint Markdown
         # Warn, don't fail yet
         run: npx markdownlint-cli *.md {specs,docs}/*.md || (echo Warning, please fix these ; true)
+      - name: Restore lychee cache
+        uses: actions/cache@v3
+        with:
+          path: .lycheecache
+          key: cache-lychee-${{ github.sha }}
+          restore-keys: cache-lychee-
+      - name: Run lychee
+        uses: lycheeverse/lychee-action@v1
+        with:
+            args: "-n --cache --max-cache-age 10d docs/rfc docs/spec"
       - name: Note any changes
         run: git status ; git diff
       - uses: ruby/setup-ruby@v1