Consolidate docs on using private Maven repositories

What does this MR do?

This change moves the documentation for connecting to private maven repositories to a single location. I moved the section to user/application_security under the section "Working in an offline environment". This change was initiated by a discussion in !27456 (diffs, comment 308352078).

Preview links:

• http://docs-preview-ee-27909.178.62.207.141.nip.io/ee/user/application_security/#using-private-maven-repos
• http://docs-preview-ee-27909.178.62.207.141.nip.io/ee/user/compliance/license_compliance/index.html#using-private-maven-repos
• http://docs-preview-ee-27909.178.62.207.141.nip.io/ee/user/application_security/dependency_scanning/#using-private-maven-repos

Related issues

• !27456 (diffs, comment 308352078)

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)
• Ping one of the technical writers for review.