Docs/AI: split AI file into two files: AI and staking

From #7262:

• [alpha only] Split AI file into two files: AI and staking
• [alpha only] Remove -new suffix from picture names
• [alpha only] Remove everything related to "AI activation", AI as a "new feature"

The new documentation can be found here:

• Staking: https://tezos.gitlab.io/-/tezos/-/jobs/7371947319/artifacts/docs/_build/alpha/staking.html
• AI: https://tezos.gitlab.io/-/tezos/-/jobs/7371947319/artifacts/docs/_build/alpha/adaptive_issuance.html

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR