Docs: Update To List Custom Analyzers

Problem to solve

• What docs or doc section affected? Include links or paths.
  • https://docs.gitlab.com/12.1/ee/user/application_security/dependency_scanning/analyzers.html
• Is there a problem with a specific document, or a feature/process that's not addressed sufficiently in docs?
  • Shows ability to customize analyzers, but does not list them.

Further details

Proposal

Who can address the issue

Other links/references