Allow skipping forks and mirrors from being indexed (#23187)

This PR adds two new options to disable repo/code search indexing of
both forks and mirrors.

Related: #22842

custom/conf/app.example.ini

@@ -1362,6 +1362,10 @@ LEVEL = Info
 ;; repo indexer by default disabled, since it uses a lot of disk space
 ;REPO_INDEXER_ENABLED = false
 ;;
+;; repo indexer units, the items to index, could be `sources`, `forks`, `mirrors`, `templates` or any combination of them separated by a comma.
+;; If empty then it defaults to `sources` only, as if you'd like to disable fully please see REPO_INDEXER_ENABLED.
+;REPO_INDEXER_REPO_TYPES = sources,forks,mirrors,templates
+;;
 ;; Code search engine type, could be `bleve` or `elasticsearch`.
 ;REPO_INDEXER_TYPE = bleve
 ;;