Update guidance on build from source to retrieve python dependencies

[MicahGale]

I was recently installing from source to review #3140 following the official guide. However, when I tried to import openmc I got an ImportError for endf. This is because I never had to run pip to install this dependency.

I think the build process was not properly updated with the migration to pyproject.toml. The documentation should add a step to include pip install ., unless this is supposed to be handled by cmake but is not being handled.

[ahnaf-tahmid-chowdhury]

The openmc Python package must be installed separately. The easiest way to install it is using pip. From the root directory of the OpenMC repository, run:

python -m pip install .

You may find this at the bottom of the documentation. However, to mitigate this issue, we have an active PR open, #3087

[MicahGale]

Oh I see that now. I think this should be in the main code block because after make install it feels like you are done so I just stop reading. (Ok I never read the comments to begin with).