[PROJ] Motion: Migrate proj.org to ReadTheDocs

[Mike Taves]

Just to add a technical note for RTS, I tried implementing RTD for
PROJ a few years ago, but ran into a blocker. There was no
hooks/ability to modify the artefacts after they were built, which is
required to publish JSON schema in a custom path. This is now overcome
with pre/post build jobs [1] available since April 2022.

Interacting with RTD for the past month has been mostly positive.
There is plenty of development flexibility, e.g. finding specific
versions of Doxygen from conda-forge. I've encountered a few random
build failures, but nothing too bad (these happen on other CI
infrastructure too).

[1] https://github.com/readthedocs/readthedocs.org/pull/9016