Update installation Docs for pip

[tylerbarna]

Feature Summary
Update the documentation to reflect nmma now being installable via pip. Would be good to integrate the changes made to the structure of the installation docs in #105 as part of solving this issue

Usage / behavior
When looking at the installation docs, they would be updated

Alternative Solutions
Just updating the docs without adding the structure changes

Implementation details
Edit the docs file on installation and update it

Additional context
This is the relevant file in the docs: https://github.com/nuclear-multimessenger-astronomy/nmma/blob/main/doc/installation.md

[sahiljhawar]

Is pip install nmma a standalone command? As in, can be used without worrying about pbilby, multinest, etc?