docs: rewrite README with structured overview and quick start guide

Replaces the minimal project description with a comprehensive README
including a component overview table, quick start instructions, common
Ansible operations, and links to detailed documentation. Aligns with
Red Panda Approval™ standards.

ansible/gitea_runner/gitea-runner.service.j2

@@ -0,0 +1,17 @@
+[Unit]
+Description=Gitea Runner
+After=network.target docker.service
+Requires=docker.service
+
+[Service]
+Type=simple
+User=gitea-runner
+Group=gitea-runner
+WorkingDirectory=/srv/gitea-runner
+ExecStart=/usr/local/bin/act_runner daemon --config /srv/gitea-runner/config.yaml
+Restart=on-failure
+RestartSec=10
+Environment=HOME=/srv/gitea-runner
+
+[Install]
+WantedBy=multi-user.target