add jupyter notebook support in documentation

[pstjohn]

Adds support for jupyter notebook rendering in the bionemo2 docs suite. This PR adds two steps:

• renders jupyter notebooks in the docs/docs/ directory tree as part of the mkdocs site using mkdocs-jupyter
• adds a new test requirement in nbval that will execute jupyter notebooks in the docs directory, make sure they succeed, and optionally compare outputs for select cells.

For the second bullet, we'll need to add a new command to the github-bionemo-ci pipeline.

[pstjohn]

Here's the resulting rendered docs page:

[jstjohn]

Woo!

[pstjohn]

/build-ci