[meta] Docs: Review and update docs for project settings

Follow up from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13552

Review https://docs.gitlab.com/ee/user/project/settings/ to identify gaps in coverage

The structure of the docs should match that of the UI, and each setting should link to further relevant docs about the feature, if they exist.

Docs missing

• [-] project's tags N/A, with the introduction of topics
• Wiki settings (enabling/disabling, any others): https://docs.gitlab.com/ee/user/project/settings/index.html#sharing-and-permissions
• project's visibility settings: https://docs.gitlab.com/ee/public_access/public_access.html
• show MR link when pushing from the command line

& Potentially further functionality, pending review.

Needs refactor/relocation

• Public availability (public, internal, private): https://docs.gitlab.com/ee/public_access/public_access.html#visibility-of-projects

Needs improvement

• Snippets setting clarification (and cross link with snippets doc) https://gitlab.com/gitlab-org/gitlab-ce/issues/64122