Docs: remove endorser note

Context

It's a proposal. I believe Ithaca is old enough so we can remove the notes about the endorser in the documentation. Note that in docs/introduction/howtouse.rst there is still a mention of the endorser, we could potentially remove it as well.

$ git grep "octez-endorser" docs/
docs/.gitignore:octez-endorser.html
docs/introduction/howtouse.rst:  note that prior to v12 of Octez, there was also an endorser daemon ``octez-endorser-*``

Manually testing the MR

See the generated documentation

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