The docs site imports docs/english/_sidebar.json and filters it; it does
not read the generated reference/sidebar.json. Replace the external
"Reference" link with the generated category tree, prefixing doc IDs with
tools/bolt-python/ so they resolve against the docs root.

The generator now does this automatically (_sync_reference_sidebar) so the
sidebar stays in sync on every regeneration.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>