CI: Convert paths-ignore into separate job that checks changed paths

We need to invoke the CI jobs every time, even if nothing relevant has
changed, because we marked them as required status checks for PRs. If
they are not invoked, the associated status checks remain in a 'pending'
state indefinitely not allowing us to (auto-)merge the PR.

.github/workflows/ci.yml

@@ -1,22 +1,40 @@
 name: CI
 
-on:
-  push:
-    paths-ignore:
-      - 'Documentation/**'
-      - '*.md'
-  pull_request:
-    paths-ignore:
-      - 'Documentation/**'
-      - '*.md'
+on: [push, pull_request]
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.head_ref || format('{0}-{1}', github.ref, github.run_number) }}
   cancel-in-progress: true
 
 jobs:
+  # Look at changed paths in the commit or PR to determine whether we need to run the complete CI job. If only
+  # documentation or .md files were changed, we wouldn't need to compile and check anything here.
+  path-changes:
+    runs-on: ubuntu-24.04
+    permissions:
+      pull-requests: read
+    outputs:
+      source_excluding_docs: ${{ steps.filter.outputs.source_excluding_docs }}
+    steps:
+      # Only need a checkout if we're not running as part of a PR
+      - uses: actions/checkout@v4
+        if: github.event_name != 'pull_request'
+
+      # FIXME: change into `dorny/paths-filter@v3` when https://github.com/dorny/paths-filter/pull/226 is merged
+      - uses: petermetz/paths-filter@5ee2f5d4cf5d7bdd998a314a42da307e2ae1639d
+        id: filter
+        with:
+          predicate-quantifier: every  # all globs below must match
+          filters: |
+            source_excluding_docs:
+              - '**'
+              - '!Documentation/**'
+              - '!*.md'
+
+  # CI matrix - runs the job in lagom-template.yml with different configurations.
   Lagom:
-    if: github.repository == 'LadybirdBrowser/ladybird'
+    needs: path-changes
+    if: github.repository == 'LadybirdBrowser/ladybird' && needs.path-changes.outputs.source_excluding_docs == 'true'
 
     strategy:
       fail-fast: false