Migrate info from the Wiki to Markdown files

What does this MR do?

This migrates the information that currently lives in GitLab UI's Wiki into Markdown files. Given that most of GitLab UI's documentation is already tracked as .md files in the repo, migrating the Wiki will ensure that we have a SSoT for all of GitLab UI's docs.

The Wiki currently has 4 pages:

Page	Actions taken
Frequently asked questions	Migrated to the FAQ.md.
GitLab UI	None. This looks like a dummy page that doesn't contain any relevant info.
Overview	Objectives have been migrated to the README.md. The Ownership section has been dropped altogether as it's not accurate anymore.
Roadmap	None. This roadmap seems outdated and is never referred to. It seems safe to remove it altogether. Note that this epic also outlines the roadmap: &950.

Related to #456 (closed)