Update the Contributor Guide

[shoeffner]

Issue

While preparing my PR for #192, I noticed that the Contributor guide is a little outdated.

• One should now use poetry to install the dev-dependencies as specified in the pyproject.toml, in contrast to the different requirements.txt files, however, poetry behaves nicely if one already has a .venv (as I learned "the hard way")
• tox -e docs does not work to build the docs, it expects a Python 3.6 which I don't have at hand and which is EOL anyways. Ubuntu 20 is still on 3.8, I think, but even the default for the tests is 3.11, so this should probably be updated. I built the docs with sphinx-build pyDataverse/docs/source build (inspired by the .readthedocs.yml) , but it couldn't find some of the rst files in the repository root. I ignored that issue as it was not relevant for me, but I'll check the tox.ini and see what one should configure instead, or what should be the working directory.
• The main branch is now main and no longer master.
• The testing section also mentions tox -py36, which should probably become -py311 (however, the tox environments are only py36,py37,py38 -- so maybe tox is just not really used anymore?)

I will update the points I mentioned here and create a PR.

[shoeffner]

I found various mentions of the master and develop branches. It looks like the develop branch fell out of favor (although it is 27 commits ahead of the main branch, it is also about a hundred commits behind).

I will update the guide to reference main (instead of master and develop), unless you have any objections.