readthedocs broken from forks

[beckermr]

I changed RTD to a simpler git checkout process to enable the version to be rendered correctly. However, this is breaking RTD builds from forks. See https://app.readthedocs.org/projects/jax-galsim/builds/32893235/

That needs fixing.

[beckermr]

I was using

git clone $READTHEDOCS_GIT_CLONE_URL .
git checkout $READTHEDOCS_GIT_IDENTIFIER || git checkout $READTHEDOCS_GIT_COMMIT_HASH

[beckermr]

A combination of random commits to main plus this PR (#256) fixed things. fun.