Update secret detection docs to new page

What does this MR do?

This breaks out the current subsection of SAST that about Secret Detection and pulls it into its own page. It also updates a lot of the content.

Related issues

#215207 (closed)

Closes

Moving docs to a new location?

Read the guidelines: https://docs.gitlab.com/ee/development/documentation/index.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ and ee/app/views (for GitLab EE) directories.
• Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
• Update the link in features.yml (if applicable)
• If working on CE and the ee-compat-check jobs fails, submit an MR to EE with the changes as well (https://docs.gitlab.com/ce/development/documentation/index.html#cherry-picking-from-ce-to-ee).
• Ping one of the technical writers for review.