Remove Home link from global navigation

What does this MR do and why?

Updates to the global nav:

• Removes the link to "Home" from the left sidebar, which is actually a link to https://docs.gitlab.com/ee, not the homepage.
• Moves the items that were underneath "Home":
  • "GitLab: The DevSecOps platform" becomes the top-level item for this section and is renamed to "About" (in the navigation only; actual page title stays the same)
  • Other pages in this section (currently this is just https://docs.gitlab.com/ee/legal/use_generative_ai.html) move under the "About" page

Why:

• It's confusing for users to have a page labeled "Home" that isn't the homepage for the website (see !4461 (comment 1729905189)).
  • The page at /ee is the homepage for /help, but the website homepage is what you see at https://docs.gitlab.com. The /ee page is not useful in the website context.
  • Users can reach the homepage by clicking the link on the logo in the top-left corner. This is standard practice; it is not standard practice to include the homepage in the sidebar navigation. In terms of IA, a homepage is a level above the top-level pages in the sidebar.
• The Hugo site drops the /ee page (see #1081 (closed), #710).
  • Updating the global nav to reflect this now saves us a step in the migration: anything we can do before the full switchover reduces complexity of rolling out the new site.

This does not remove the /ee page from the site. That page should still exist while the other pages have the ee/ prefix in their URLs. This only changes the left sidebar. The ee/ prefix will be fully dropped on the Hugo site.

Screenshots, screen recordings, or links to review app

Before	After

Review app: https://nav-home-link.docs.gitlab-review.app/ee/devsecops.html

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