docs: move CLI configuration content to the docs site

Description

Moves the README Configuration section to the docs site as cli/configuration/: configuration levels, the configuration search order, and platform-specific file locations.

The README keeps a pointer. The Configuration heading stays, so existing #configuration anchor links still resolve.

Three references to the removed README anchor now point at the new page: two in docs/source/authentication.md, and the glab config long help in internal/commands/config/config.go that generates the same line in docs/source/config/_index.md. CI runs lychee --offline, so a rotted link to gitlab.com here would not have been caught.

The note uses Markdown alert syntax, which gitlab_base.AlertFormat requires at error level on docs/source/.

Related issues and merge requests

Related to #8486 (closed). Follows !3692 (merged) (authentication) and !3699 (merged) (commands).

Part 1 of 3. The navigation entry for cli/configuration/ is a separate MR in docs-gitlab-com, which can only merge once this page is on main.

Updated navigation (draft)