Update Copy for Self-Hosted Models UI

What does this MR do and why?

Update copy for self-hosted models pages so that they are consistent with GitLab preferred styles. The changes were flagged by a technical writer.

Links to: #466605 (closed)

This MR:

• Updates occurrences of "Self-Hosted Models" copy on buttons, titles and descriptions to use sentence case ("Self-hosted models")
• Updates occurrences of "AI-Powered Features" to use sentence case ("AI-powered features"). This includes the admin top-level menu item.
• Removes "Pro" from "GitLab Duo Pro" in the admin top-level menu item
• Updates documentation to reflect above changes
• Adds namespacing to localisation where it had been previously missed

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Admin Area Menu

Before	After

Models Page

Before	After

Models > Edit Page

Before	After

Models > New Page

Before	After

How to set up and validate locally

Prequisites

You will need:

• An active GitLab ultimate license
• Disable SAAS mode when running GDK
  • echo $GITLAB_SIMULATE_SAAS should return 0
  • See the docs for more information
• Run the following commands in your terminal

add_on = GitlabSubscriptions::AddOn.find_or_create_by!(name: "code_suggestions") {|e| e.description = "Test"}
add_on_purchase = GitlabSubscriptions::AddOnPurchase.create!(add_on: add_on, expires_on: 1.month.from_now, quantity: 5, purchase_xid: 'A-S0001')
Feature.enable(:self_managed_code_suggestions)
Feature.enable(:ai_custom_model)
ApplicationSetting.first.update(duo_features_enabled: true)

1. Navigate to your GDK instance in browser
2. Click on "Admin" button at the bottom of the left sidebar
3. Open the "AI powered Features" drop down in the admin area sidebar
4. Click on "Models"
5. You may may be asked to accept some terms and conditions with usage (accept them)
6. If you do not have any models set up, you will see an empty state where you will be prompted to create a new one
7. Create new models. You should see a list of models now.
8. Validate the copy is updated on this page
9. Also add and edit a model to validate those pages as well.

Numbered steps to set up and validate the change are strongly suggested.