Add docs to metadata for gatekeeper

Gatekeeper is using the `main` branch to fetch the charmhub url to the charm's doc set. This cause CI in PRs to fail because no PR has been merged to update this field

metadata.yaml

@@ -14,7 +14,7 @@ display-name: Charm Template
 
 # (Required)
 summary: A very short one-line summary of the charm.
-docs: https://discourse.charmhub.io
+docs: https://discourse.charmhub.io/t/jenkins-agent-documentation-overview/12694
 issues: https://github.com/canonical/is-charms-template-repo/issues
 maintainers: 
   - https://launchpad.net/~canonical-is-devops