UI text updates for custom models

This issue is to track some proposed changes to UI text.

Left menu - should be sentence case

The top-level menu item should be AI-powered features (sentence case):

Models top page

• Breadcrumb should be Self-hosted models
• Change this phrase:
  • From They point to the self-hosted AI models that can be used for backing up GitLab AI features
  • To: Add self-hosted language models to use as backups for GitLab Duo features
• Button should be New self-hosted model

New model page

• Breadcrumb should be New self-hosted model
• Title should be New self-hosted model
• Button should be Create model