docs: Document XDG Base Directory support for glab config

What does this MR do?

Adds comprehensive XDG Base Directory information to the glab config command help text and updates the generated documentation site to include the new section. The help now explains the config file search order, platform‑specific defaults, legacy location handling, and the GLAB_CONFIG_DIR override.

Why

Addresses Issue #8332 (closed) by providing users with clear guidance on where glab looks for configuration files and how the XDG specification is applied, improving usability and reducing confusion.

Testing

Run glab config --help and verify that the XDG Base Directory section appears as expected. Also build the docs (make gen-docs) and confirm the new content is rendered in docs/source/config/_index.md.

Closes #8332 (closed)