GitHub Actions: Add a labeler action

Use the following GitHub action to automatically label pull requests
based on the paths of the modified files:
https://github.com/marketplace/actions/labeler

Use it to label pull requests introducing new status reports with the
'status report' label.

Approved by:	uqs (github-automation), dbaio (doceng)
Pull Request:	https://github.com/freebsd/freebsd-doc/pull/121

.github/conf/label-pull-requests.yml

@@ -0,0 +1,2 @@
+status report:
+- website/content/en/status/**

.github/workflows/label-pull-requests.yml

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+name: "Label pull requests"
+on: [pull_request_target]
+
+jobs:
+
+  label-pull-requests:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+    - uses: actions/labeler@v4
+      with:
+        repo-token: "${{ secrets.GITHUB_TOKEN }}"
+        configuration-path: ".github/conf/label-pull-requests.yml"