feat: add GitHub workflows for Markdown links checking

Two GitHub workflows have been added that perform Markdown links
checking:
* .github/workflows/markdown-link-check-pr.yaml: Executed
  in each PR.
* .github/workflos/markdown-link-check-weekly.yaml: Executed
  Weekly on Mondays at 07:00 UTC.

All the specified md files and directories in the workflows are
intentionally checked, independently on whether they have been
modified or not. The reason for this is that even if a given .md
file has not been modified, its links can become broken. For example,
if a PR moves .md files from directories and some other .md files
contain relative links referencing the old path of the moved files,
then the links become broken even though the file containing the
references not being modified.

.github/workflows/markdown-link-check-pr.yaml

@@ -0,0 +1,38 @@
+name: PR check Markdown links
+
+# Only run this workflow on PRs
+on:
+  pull_request:
+    types: [opened, synchronize, reopened, ready_for_review]
+
+# Disable permissions for all available scopes for the GITHUB_TOKEN
+# except for the metadata scope.
+permissions: {}
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  pr-markdown-link-check:
+    # Only run the job on non-draft PRs
+    if: "github.event.pull_request.draft == false"
+    name: Check PR Markdown links
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out code under the $GITHUB_WORKSPACE directory
+        uses: actions/checkout@v3
+      - name: Check markdown links
+        uses: gaurav-nelson/[email protected]
+        with:
+          use-quiet-mode: 'no'
+          use-verbose-mode: 'yes'
+          config-file: './.markdown-link-check/markdown-link-check-config.json'
+          check-modified-files-only: 'no'
+          file-extension: '.md'
+          # Unfortunately there's currently not a way to ignore specific
+          # directories/paths in the GitHub action so we need to specify
+          # the desired set of directories/paths that we want to check
+          # in order to avoid checking undesired directories/paths
+          folder-path: './docs, ./pkg, ./templates, ./.github, ./cmd, ./internal, ./openapi, ./docker, ./scripts'
+          file-path: './README.md, ./CONTRIBUTING.md'

.github/workflows/markdown-link-check-weekly.yaml

@@ -0,0 +1,32 @@
+name: Weekly check Markdown links
+
+# Run this workflow every Monday at 07:00 UTC
+on:
+  schedule:
+    - cron: "0 7 * * 1"
+
+# Disable permissions for all available scopes for the GITHUB_TOKEN
+# except for the metadata scope.
+permissions: {}
+
+jobs:
+  weekly-markdown-link-check:
+    name: Weekly Check Markdown links
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out code under the $GITHUB_WORKSPACE directory
+        uses: actions/checkout@v3
+      - name: Check markdown links
+        uses: gaurav-nelson/[email protected]
+        with:
+          use-quiet-mode: 'no'
+          use-verbose-mode: 'yes'
+          config-file: './.markdown-link-check/markdown-link-check-config.json'
+          check-modified-files-only: 'no'
+          file-extension: '.md'
+          # Unfortunately there's currently not a way to ignore specific
+          # directories/paths in the GitHub action so we need to specify
+          # the desired set of directories/paths that we want to check
+          # in order to avoid checking undesired directories/paths
+          folder-path: './docs, ./pkg, ./templates, ./.github, ./cmd, ./internal, ./openapi, ./docker, ./scripts'
+          file-path: './README.md, ./CONTRIBUTING.md'