Fix issue with building Sphinx documentation with recent Jinja version

[hakonsbm]

Fixes #2038 by adding a requirement to keep Jinja version below 3 when creating a Conda environment. Also adds the requirement for Read the Docs.

[terhorstd]

I am very unhappy with adding transitional dependencies to the nest-dev environment. Now it looks like NEST depends on an old version of Jinja2, where actually this is a bug in the Sphinx package not stating its dependencies correctly and NEST itself doesn't have any direct interest in Jinja2 at all.

For the time being I agree to add it, in order to prevent everyone's doc generation workflow to break. I could imagine this gets fixed upstream in sphinx/nbformat soon and we should remove the whole jinja2 dependency again. (please create an issue)

[hakonsbm]

@terhorstd I agree that it's not optimal. Like I wrote in the issue, Jinja 3.0 support will be added with Sphinx 4.1.0, which, according to their milestones, should be finished June 12.

[jessica-mitchell]

Thanks for catching this @hakonsbm. I just want to point out, at least up to now there have been no problems building on RtD, but since a fresh install of the conda environment includes jinja2 3.0 I agree with @terhorstd that this should be approved to not break anyone's workflow.