docs: move CLI connection content to the docs site

Description

Moves three README subsections to the docs site as cli/connection/: pointing glab at a GitLab Self-Managed or GitLab Dedicated instance, mTLS certificates, and self-signed certificates.

The README keeps a pointer, and the three table of contents entries are removed.

Related issues and merge requests

Related to #8486 (closed).

Part 2 of 3, stacked on !3813 (merged) and targeting its branch. Review !3813 (merged) first. The navigation entry for cli/connection/ is a separate MR in docs-gitlab-com, which can only merge once this page is on main.

Updated navigation (draft)