Editorial: Update ATK URL

[smockle]

Initially filed as https://github.com/w3c/html-aam/pull/560

https://gnome.pages.gitlab.gnome.org/atk/ (the URL currently referenced throughout html-aam) 404s, even after logging in to the GNOME GitLab instance[^1].

~https://docs.gtk.org/atk/ (this PR’s replacement URL) seems to be the new location. GTK’s “API Docs” page links to this URL.~ Update: There are 3 candidates; details in https://github.com/w3c/aria/pull/2309#issuecomment-2294105902.

Preview: https://deploy-preview-2309--wai-aria.netlify.app/html-aam/

[^1]: That said, GNOME’s “Technologies” page still links to this URL. 🤷

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Test, Documentation and Implementation tracking

Once this PR has been reviewed and has consensus from the working group, tests should be written and issues should be opened on browsers. Add N/A and check when not applicable.

• [ ] "author MUST" tests:
• [ ] "user agent MUST" tests:
• [ ] Browser implementations (link to issue or commit):
  • WebKit:
  • Gecko:
  • Blink:
• [ ] Does this need AT implementations?
• [ ] Related APG Issue/PR:
• [ ] MDN Issue/PR: