Updated documentation generator for custom roles

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What does this MR do and why?

• Updated the docs generator for custom roles to a new format (as described in #482752)
• Compiled custom roles docs
• Removed 2 unnecessary methods in docs generator for custom roles

Screenshots or screen recordings

How to set up and validate locally

• Run bundle exec rake gitlab:custom_roles:compile_docs to compile the docs
• Run Gitlab Docs in GDK

Related to #482752