docs: move CLI proxy content to the docs site

Description

Moves the authenticating-proxy content to the docs site as cli/proxies/: the custom_headers configuration and the Google Identity-Aware Proxy example. Giving provider-specific examples their own page stops them accumulating in a README subsection.

The README keeps a pointer, and the table of contents entry is removed.

This is a straight move. The transparent SSH ProxyCommand configuration scoped in #8486 (closed) never landed in the README, so it needs writing and testing against a real IAP instance rather than moving. It follows separately.

Related issues and merge requests

Related to #8486 (closed).

Part 3 of 3, and the last cli MR for the issue. !3813 (merged) and !3814 (merged) have merged, so this now targets main directly. main was merged in to resolve the README conflict the connection page created.

The navigation entry for cli/proxies/ is in a separate docs-gitlab-com MR, !2584 (merged), which stays in Draft until this page is on main.

Updated navigation (draft)