Updates nav for Administer section

What does this MR do and why?

This MR updates the categories in the left navigation/IA for the Administer docs.

It moves self-hosted models under Configure. Why? The self-hosted model docs:

1. Are about configuring a specific GitLab feature (GitLab Duo).
2. Involve setting up and managing AI models within an existing self-managed GitLab installation.

This MR also updates the Administer landing page to align.

Review app

Left nav changes

• Before: https://docs.gitlab.com/17.5/ee/administration/self_hosted_models/
• After: https://lyspin-administer-nav-titles.docs.gitlab-review.app/ee/administration/self_hosted_models/

Administer landing page changes

• Before: https://docs.gitlab.com/17.5/?tab=Administer
• After: https://lyspin-administer-nav-titles.docs.gitlab-review.app/?tab=Administer

Related MR: gitlab!172885 (comment 2214290449)