Improve the Transaction Rollup documentation with the Phase 2

Context

This is a doc-only MR that aims to bring the most recent version of the “Transaction Rollup” to master. The rationale is that it is way better than the current version, that is actually online right now on tezos.gitlab.io. Therefore, I think it’s a good idea to improve it, in case interested people want to read it before the end of the development.

This version has been reviewed by @NicVolanschi and @eugenz in !3921 (closed); their comments probably needs to be taken into account before merging. Hence the Draft status.

Manually testing the MR

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, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR