Add link aliveness tests

Signed-off-by: Daniel Farrell <dfarrell@redhat.com>

.github/ISSUE_TEMPLATE/broken-link.md

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+---
+name: Broken link detected by CI
+labels: bug
+---
+
+<!-- Used by automation to raise an Issue when the periodic link aliveness tests detect a broken link. -->
+
+Periodic link aliveness CI detected a broken link. Please see the job results for details.

.github/workflows/md-links-periodic.yml

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+name: Markdown Link Checks
+on:
+  schedule:
+    - cron: "0 0 * * 0"
+
+jobs:
+  markdown-link-check-periodic:
+    runs-on: ubuntu-latest
+    name: Markdown Link Checks
+    steps:
+    - name: Check out code
+      uses: actions/checkout@v1
+
+    - name: Run markdown-link-check
+      uses: gaurav-nelson/github-action-markdown-link-check@0.6.0
+      with:
+        config-file: ".markdownlinkcheck.json"
+
+    - name: Raise an Issue to report broken links
+      if: ${{ failure() }}
+      uses: JasonEtco/create-an-issue@v2
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      with:
+        filename: .github/ISSUE_TEMPLATE/broken-link.md

.github/workflows/md-links.yml

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+name: Markdown Link Checks
+on: pull_request
+
+jobs:
+  markdown-link-check:
+    runs-on: ubuntu-latest
+    name: Markdown Link Checks
+    steps:
+    - name: Check out code
+      uses: actions/checkout@v1
+
+    - name: Run markdown-link-check
+      uses: gaurav-nelson/github-action-markdown-link-check@0.6.0
+      with:
+        config-file: ".markdownlinkcheck.json"
+        check-modified-files-only: "yes"