Currently, we include the source codes of the Sphinx theme sphinx_rtd_theme (https://github.com/GenericMappingTools/gmt/tree/master/doc/rst/themes/rtd) and some Sphinx extensions (https://github.com/GenericMappingTools/gmt/tree/master/doc/rst/_extensions) in the GMT repository.

I think the main reasons to include them in GMT are:

1. Developers/users don't have to install these Python packages to build the GMT documentation
2. They're under our control so the documentation won't suddenly break [Actually it still breaks sometimes due to changes in Sphinx].

However, including them in GMT also have some downsides:

1. The size of the GMT source tarballs increases by about 3 MB.
2. We have to maintain them manually (xref Add maintenance instructions for sphinx extensions/themes #6010)
3. We can't use some good Sphinx extensions (e.g., https://sphinx-design.readthedocs.io/en/latest/) because their source codes are complicated or too big to be included in the repository

Since all the Sphinx packages can be installed using pip and pip itself is also easy to install,
what about removing them from the GMT repository?