doc: have the 'OCaml flags' documentation point to complete examples

[gasche]

I had forgotten how to disable a warning with Dune, unfortunately the "OCaml flags" documentation does not actually contain any reusable code snippet doing this. In fact, the information is in the doc, hidden in the "Quickstart" and the "FAQ" pages.

This commit adds references from the "OCaml flags" reference section to those usage examples.

[gasche]

(I haven't tried generating the documentation locally to check that the reference syntax works. Hopefully someone who knows about this stuff can check from the CI results.)

[Alizter]

There is a docs/readthedocs.org:dune job in the CI that you can use to view the documentation.

Edit: For some reason, the readthedocs is failing, even if I manually go to the job.