Add htmlproofer CI workflow (advisory)

Adds htmlproofer to the CI pipeline to catch broken internal links on
every PR and push to master. Runs in advisory mode (continue-on-error:
true) due to inherited link debt from the Puppet docs migration.

- Internal links only (--disable-external) to avoid third-party flakiness
- Ignores legacy /puppet/, /pe/, /puppetlabs/ URL prefixes
- Pinned to ubuntu-24.04 and actions/checkout@v4 to match test.yml

Signed-off-by: Michael Harp <mike@mikeharp.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: miharp

.github/workflows/htmlproofer.yml

@@ -0,0 +1,31 @@
+---
+name: HTML Proofer
+
+on:
+  pull_request: {}
+  push:
+    branches:
+      - master
+  workflow_dispatch: {}
+
+permissions:
+  contents: read
+
+jobs:
+  html-proofer:
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.3'
+          bundler-cache: true
+      - name: Build site
+        run: bundle exec jekyll build
+      - name: Check links (advisory — failures reported but do not block)
+        continue-on-error: true
+        run: |
+          bundle exec htmlproofer _site \
+            --disable-external \
+            --ignore-urls "/puppet/,/pe/,/puppetlabs/" \
+            --allow-hash-href

Gemfile

@@ -10,6 +10,7 @@ gem 'rake', '~> 13.0', '>= 13.0.1'
 gem 'versionomy', '~> 0.5.0'
 
 group(:build_site) do
+  gem 'html-proofer', '~> 5.0'
   gem 'jekyll', '~> 4.4'
   gem 'jekyll-vitepress-theme', '~> 1.4'
 end