Write docs on getting started

[matentzn]

These docs are massively outdated:

https://mapping-commons.github.io/sssom-py/installation.html

We need to have a concise guide on how to get set up for new developers.

maybe @joeflack4 after you figured it out, you can add the steps here or make a PR on the docs?

Something like "Getting started for sssom-py developers"

And then the steps needed to set it up.

[joeflack4]

The main thing I would change is installing from tox rather than `requirements.txt.