Add Jinja2 dependency to docs and pin to 3.0.3

[ilumsden]

Our docs depend on nbsphinx, which in turn depends on nbconvert. Unfortunately, there is an issue related to nbconvert that causes our docs to fail if a version of Jinja2 greater than 3.0.3 is installed.

So, we need to add a dependency on Jinja2 3.0.3 to docs/requirements.txt.