Define a process to test the readthedocs CI before merging into master #3369

This is an offshoot of #3358.

The readthedocs CI has a bad habit of failing even after the Azure Pipelines job "Docs" has succeeded.

After major changes that impact the documentation, and before merging everything into master, it would be advisable to explicitly verify that RTD builds correctly.

So far I tried to

1. create my own readthedocs project, https://readthedocs.org/projects/crusaderky-xarray/
2. point it to my fork https://github.com/crusaderky/xarray/
3. enable build for the branch I want to merge

This is currently failing because of an issue with versioneer, which incorrectly sets xarray.__version__ to 0+untagged.111.g6d60700.
This in turn causes a failure in a minimum version check in pandas.DataFrame.to_xarray() on pandas>=0.25.

In the master RTD project https://readthedocs.org/projects/xray/, I can instead read xarray: 0.13.0+20.gdd2b803a.

So far the only workaround I could find was to downgrade pandas to 0.24 in ci/requirements/doc.yml.