use sphinx_rtd_theme

[jburel]

Set rtd theme

[sbesson]

This seems to fix the theme regression issue that has been observed between https://omero.readthedocs.io/en/v5.6.8-2/ and https://omero.readthedocs.io/en/v5.6.8-3/.

Is there an official link describing this as a the new canonical way to declare the RTD theme?
Will this be a change to apply across all rtd repositories (i.e. lots of upcoming noise)?

[jburel]

We will have to propagate across all repos.
I have not seen a page
But RTD might have dropped it at some point because it did not have support for Sphinx 7 see readthedocs/sphinx_rtd_theme#1463

The issue is now solved

[sbesson]

I believe the breaking change was introduced in https://docs.readthedocs.io/en/stable/changelog.html#version-10-2-0 and more specifically readthedocs/readthedocs.org#10638

[jburel]

We are not installing sphinx-rtd-theme automatically on new projects. However, we are setting sphinx_rtd_theme automatically on their docs/conf.py.

This did not happen for us since we are using omero/conf.py (even if it is not a new project) so some of our repos will be okay

[snoopycrimecop]

Empty PR description. Please add a short summary of the PR scope and some testing instructions.