docs: Pin jinja2 to < 3.1.0

docs: Pin jinja2 to < 3.1.0

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:
- https://github.com/pallets/jinja/issues/1631
- https://github.com/readthedocs/readthedocs.org/issues/9037

and

- https://github.com/PowerDNS/pdns/pull/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))
```