[Documentation] docs/requirements.txt is out-of-sync with what CI builds use

[asfimport]

docs/requirements.txt pins Sphinx 2.4.4 but the CI pipelines use the latest versions of breathe/sphinx/etc.

Reporter: David Li / @lidavidm

_{Note: This issue was originally created as ARROW-13354. Please see the migration documentation for further details.}

[amoeba]

I think we can close this because this is no longer the case as of 3f7f245 and the local developer setup is in sync with the setup in CI.