Switch to pip install in documentation. Ignore build directory.

[cmarmo]

Dear maintainer(s),

while installing for the first time the framework on a Linux system (Fedora) following the documentation I faced some issues.

• the python setup.py install does not allow to install if the installation is not performed with sudo access (as expected)
• when creating a fresh virtual environment without installing the dependencies, astropy 5.0 is downloaded and it complains about the extension_helpers not being installed (related to the deprecation of the astropy_helpers module).

To avoid these issues I'm proposing to modify the documentation and use

pip install .

This allows to install without sudo rights even without virtual environment and it also manages the dependencies correctly.

In addition, I have added a .gitignore specific to the documentation directory.

Thanks for considering this pull request!

[MNBrod]

Yup! These are changes I have been thinking about for a while but never got around to. I will merge this in with the next release.