rec-4.4.x docs: Pin jinja2 to < 3.1.0

[Habbie]

backport of #11449

Jinja2 3.1.0 removed deprecated code that is still used by sphinx
1.8.x, and it looks like our custom sphinx extensions are not working
with more recent versions of sphinx..

See:

• jinja2 3.1.0 breaks mkdocs pallets/jinja#1631
• RDT build suddenly starts failing without any changes in conf.py readthedocs/readthedocs.org#9037

and

• Require sphinx < 2.0 #7712

The exact error is:

Extension error:
Could not import extension sphinx.builders.latex (exception: cannot import name 'contextfunction' from 'jinja2' (/dnsdist/pdns/dnsdistdist/.venv/lib/python3.7/site-packages/jinja2/__init__.py))

(cherry picked from commit 92ad297)

Short description

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master