Add GHA that checks links (using lychee). Incl list of links to ignore.

.github/workflows/link-checker.yml

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+# This Link Checker is run on all documentation files once per week.
+
+# references:
+# - https://github.com/lycheeverse/lychee-action
+# - https://github.com/lycheeverse/lychee
+
+name: Link Checker
+
+on:
+  # TODO remove before merging.
+  # just using the push and pull_request event in for testing only.
+  push:
+    branches:
+      - develop
+  pull_request:
+    branches:
+      - develop
+  schedule:
+    # * is a special character in YAML so you have to quote this string
+    - cron:  '30 8 * * 5'
+
+jobs:
+  linkChecker:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Restore lychee cache
+        uses: actions/cache@v3
+        with:
+          path: .lycheecache
+          key: cache-lychee-${{ github.sha }}
+          restore-keys: cache-lychee-
+
+      - name: Link Checker
+        uses: lycheeverse/lychee-action@v1.5.2
+        with:
+          args: --verbose --no-progress --cache --max-cache-age 1d packages/mermaid/src/docs/**/*.md README.md README.zh-CN.md
+          fail: true
+          jobSummary: true
+        env:
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}

.lycheeignore

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+# These links are ignored by our link checker https://github.com/lycheeverse/lychee
+# The file allows you to list multiple regular expressions for exclusion (one pattern per line).
+
+# Network error: Forbidden
+https://codepen.io
+
+# Network error: The certificate was not trusted
+https://mkdocs.org/
+https://osawards.com/javascript/#nominees
+https://osawards.com/javascript/2019