fix: add manual dispatch to docs

Author: wdconinc

docs/build-pipeline.md

@@ -166,6 +166,7 @@ Pre-built binaries are stored in OCI registries:
 | Trigger | Behavior |
 |---------|----------|
 | Pull Request closed | Automatically removes all buildcache tags associated with the branch slug (typically unique to the PR) from ghcr.io and eicweb.phy.anl.gov |
+| Manual Dispatch | Allows manual cleanup of buildcache tags for a specific branch or PR by specifying a custom `ref_slug` parameter |
 
 ## Environment Matrix