[CI] Add link checker (#1875)

Adding link checker to the CI (closes
https://github.com/paritytech/polkadot-sdk/issues/993). It would be nice
to have the docs people own this and extend accordingly.
Currently all known-bad links are excluded, but we should one-by-one
include those as well until all are fixed.

This check now ensures that 1) no new broken links are introduced into
`.rs` files and 2) that no old links break unnoticed.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

.github/workflows/check-links.yml

@@ -0,0 +1,40 @@
+name: Check links
+
+on:
+  pull_request:
+    paths:
+      - "*.rs"
+      - "*.prdoc"
+      - ".github/workflows/check-links.yml"
+      - ".config/lychee.toml"
+    types: [opened, synchronize, reopened, ready_for_review]
+
+permissions:
+  packages: read
+
+jobs:
+  link-checker:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Restore lychee cache
+        uses: actions/cache@704facf57e6136b1bc63b828d79edcd491f0ee84 # v3.3.2 (7. Sep 2023)
+        with:
+          path: .lycheecache
+          key: cache-lychee-${{ github.sha }}
+          # This should restore from the most recent one:
+          restore-keys: cache-lychee-
+
+      - uses: actions/checkout@8ade135a41bc03ea155e62e844d188df1ea18608 # v4.1.0 (22. Sep 2023)
+
+      - name: Lychee link checker
+        uses: lycheeverse/lychee-action@2ac9f030ccdea0033e2510a23a67da2a2da98492 # for v1.8.0 (15. May 2023)
+        with:
+          args: >-
+            --config .config/lychee.toml
+            --no-progress
+            './**/*.rs'
+            './**/*.prdoc'
+          fail: true
+        env:
+          # To bypass GitHub rate-limit:
+          GITHUB_TOKEN: ${{secrets.GITHUB_TOKEN}}