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)

doc/development/permissions/custom_roles.md

@@ -169,7 +169,7 @@ For example, you see in `GroupPolicy` that there is an ability called
 than adding a row to the `member_roles` table for each ability, consider
 renaming them to `read_security_dashboard` and adding `read_security_dashboard`
 to the `member_roles` table. This is more expected because it means that
-enabling `read_security_dashboard` on the parent group will enable the custom
+enabling `read_security_dashboard` on the parent group will enable the custom role.
 For example, `GroupPolicy` has an ability called `read_group_security_dashboard` and `ProjectPolicy` has an ability
 called `read_project_security_dashboard`. If you would like to make both customizable, rather than adding a row to the
 `member_roles` table for each ability, consider renaming them to `read_security_dashboard` and adding