Test link-checker-gpt

This PR implements:

* https://github.com/kingdonb/link-checker-gpt/tree/v1-beta#integration-as-a-ci-check

I'm testing the new Dockerized version of the link-checker-gpt, that
helped us fix all the broken links in #1573

It should be possible to merge this if we are happy with it, independent
of the release for Flux v2.1 (it can certainly wait until after 👍)

I'm going to push a broken link as a test.

There has been some thought put into making a good workflow in a way
such that we don't want to 86 it right away... minor annoyances should
all be squashed prior to their annoying anyone

* add the workflow to .github/workflows

* fix the bug (upstream)

* provide GH_TOKEN to link checker
* (fix github token)

Signed-off-by: Kingdon Barrett <kingdon@weave.works>

.github/workflows/link-checker.yml

@@ -0,0 +1,26 @@
+name: Link Checker
+
+on:
+  pull_request:
+    branches:
+      - main
+
+        # eventually: (to write comments)
+# permissions:
+#   pull-requests: write
+
+jobs:
+  check-links:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v3
+
+    - name: Link Checker GPT
+      uses: kingdonb/link-checker-gpt@v1-beta # (the v1 tag is still unreleased, we need to test)
+      with:
+        productionDomain: fluxcd.io
+        previewDomain: deploy-preview-${{ github.event.pull_request.number }}--fluxcd.netlify.app
+        prNumber: ${{ github.event.pull_request.number}}
+        githubToken: ${{ secrets.GITHUB_TOKEN }}