CIAO: generation check

What

Adds a check in CIAO (CI Assembled in OCaml) verifying that all files are either generated from CIAO or part of a whitelist.

Also sneaks in unrelated documentation fixes.

Stack

• !12377 (merged): Generation check <- You are here
• !12378 (merged): before_merging/scheduled -- scaffolding
• !12382 (merged): before_merging/scheduled -- sanity, packaging
• !12490 (merged): before_merging/scheduled -- test (1)

Why

Henceforth, we want all CI files to be written in CIAO and this checks makes developers aware of this. It also makes sure we're not leaving any unused CI files behind.

How

This code is copied from manifest that has similar check for dune/opam files.

Manually testing the MR

Run make -C ci, it should report no errors. Then touch .gitlab/ci/jobs/build/foobar.yml. Now the previous command should complain. Try source scripts/version.sh; dune exec ci/bin/main.exe -- --remove-extra-files and check that the touched file is removed.

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