Add first two ActivityPub pages to nav

Add navigation item to global navigation

• Link to the merge request that introduced the new pages:
  • https://docs.gitlab.com/ee/development/activitypub/
  • https://docs.gitlab.com/ee/development/activitypub/actor.html
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

Related links

• Closes Documentation main page for the ActivityPub API (gitlab#423286 - closed)
• Related to Add documentation for ActivityPub releases (gitlab!130960 - merged) which introduced the pages
• Related to Follow-up docs questions for ActivityPub (gitlab#424420)
• Related to Add ActivityPub Releases profile and outbox (gitlab!127023 - merged)