Enable Mkdocs validation (#1383)

* Warn unrecognized_links and anchors

* Run mkdocs build --strict

https://www.mkdocs.org/user-guide/configuration/#validation

* Trigger the workflow by current file changes

* Install mkdocs only

* Revert "Install mkdocs only"

This reverts commit 57d1c71.

Author: Goooler

.github/workflows/links.yml

@@ -8,11 +8,13 @@ on:
       - docs/**
       - README.md
       - RELEASING.md
+      - .github/workflows/links.yml
   pull_request:
     paths:
       - docs/**
       - README.md
       - RELEASING.md
+      - .github/workflows/links.yml
   workflow_dispatch:
 
 jobs:
@@ -25,6 +27,11 @@ jobs:
           cache-read-only: true
       # This is required for action-linkspector to work with API links.
       - run: ./gradlew dokkaHtml
+      - name: Mkdocs build validation
+        run: |
+          # Don't cache it to track updates.
+          pip install mkdocs-material
+          mkdocs build --strict
       - uses: umbrelladocs/action-linkspector@v1
         with:
           reporter: github-pr-check

mkdocs.yml

@@ -77,3 +77,7 @@ nav:
   - 'Changes': changes/README.md
   - 'API': api/index.html
   - 'About': about/README.md
+
+validation:
+  unrecognized_links: warn
+  anchors: warn