[#4] Use xrefcheck to check links on CI

Problem: Markdown files contain a plenty of links which
tend to get outdated occasionally. It's hard to notice
when a certain link (local or global) becomes broken.

Solution: in the past, the `xrefcheck` tool was used
to find broken links:
* #3
* #454

See https://github.com/serokell/xrefcheck
This PR automates `xrefcheck` running by adding it as a new
GitHub Action job.
It uses [xrefcheck-action](https://github.com/serokell/xrefcheck-action)
under the hood.

Resolves #4

.github/workflows/checks.yml

@@ -108,3 +108,14 @@ jobs:
 
     - name: Diff plans
       run: GH=1 ./scripts/release/cabal-plan-diff.sh
+
+  xrefcheck:
+
+    name: Check references
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v4
+    - uses: serokell/xrefcheck-action@v1
+      with:
+        xrefcheck-version: 0.2