Generate custom abilities documentation

What does this MR do and why?

It generates available custom abilities documentation by running bundle exec rake gitlab:custom_roles:compile_docs

Follow-up issue: Add verification that custom abilities doc is u... (#437662 - closed)

Closes #434352 (closed)

config/feature_flags/development/manage_project_access_tokens.yml

@@ -6,3 +6,4 @@ milestone: '16.5'
 type: development
 group: group::authorization
 default_enabled: false
+feature_flag: manage_project_access_tokens