docs: mention Forgejo in the installation overview

Forgejo support exists, but the installation overview did not mention
it. Add it so the docs match the providers the project supports.

Author: chmouel

docs/content/docs/installation/overview.md

@@ -26,3 +26,4 @@ After installing Pipelines-as-Code, configure your Git provider to send webhook
 * [GitLab]({{< relref "/docs/providers/gitlab" >}})
 * [Bitbucket Data Center]({{< relref "/docs/providers/bitbucket-datacenter" >}})
 * [Bitbucket Cloud]({{< relref "/docs/providers/bitbucket-cloud" >}})
+* [Forgejo]({{< relref "/docs/providers/forgejo" >}})