docs: add mkdocs config and CI for building infra-docs website

Heavily inspired by: https://forgejo.c3voc.de/voc/av-docs

.forgejo/workflows/cleanup-docs.yaml

@@ -0,0 +1,24 @@
+on:
+  pull_request:
+    types:
+      - closed
+
+jobs:
+  cleanup-staging:
+    runs-on: docker
+    container:
+      image: code.forgejo.org/oci/node:22-bookworm
+    steps:
+      - name: Pipeline info PR
+        run: |
+          echo "Run triggered by ${{ github.event_name }} (${{ github.event.action }}) on ref ${{ github.ref_name }}"
+
+      - name: Staging Deployment - Prepare keys
+        run: |
+          echo "${{ secrets.SSH_DEPLOY_KEY }}" > deploykey.priv
+          chmod 400 deploykey.priv
+          echo "${{ vars.SSH_KNOWN_HOSTS }}" > ./known_hosts
+
+      - name: Staging Deployment - Remove PR from staging
+        run: |
+          ssh -i deploykey.priv -o 'UserKnownHostsFile ./known_hosts' infra-docs-deploy@public-web-static.hosts.hamburg.ccc.de -t "rm -r /var/www/staging.infra-docs.hamburg.ccc.de/pr${{ github.event.pull_request.number }}/"