Manifest: generate dune files in [docs/doc_gen] and simplify [rpc_doc.ml]

Context

Use manifest to generate the dune files in docs/doc_gen.

Notes:

• it is safe to remove -safe-string, it being already the default. moreover, this option becomes redundant with ocaml5.
• the logic for the set of embedded protocols to link into rpc_doc and p2p_doc is not wholly clear to me, and should ideally be clarified in a comment or in the issue that is referred to in the code.

In the third commit, we also simplify rpc_doc.ml so that it no longer needs to maintain a list of protocols. This simplifies the work outlined in #2170.

Manually testing the MR

• Compare the old / generated dune files.
• Check that the generated documentation looks ok.

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