README and doc

[junziz]

Add a new brief README:
[DONE] Add README based on draft intro and create a MWE with the latest signature.
[DONE] Add some additional badges for documentation, test coverage and CI/build passing, etc. Some good examples for these additional badges: https://github.com/cvxgrp/scs/blob/master/README.md and https://github.com/cvxgrp/pymde.
[DONE] Add references to arXiv once it's there. Add it to the citing parts (of README and doc) and also the end of the usage part.

Documentation:
[] Add intro to MFG and when and how to use MFG. Edit NE and exploitability part — currently Gamma is not defined.
[] Polish the explanations on MF-OMO, especially the "more on MF-OMO" part.
[] Show verbose printout examples. MF-OMO verbose printout should provide optimizer info?
[] Describe how to uninstall using poetry for developers when development is done. Also what happens when mistakenly installing using poetry again before uninstalling the old one in a new/updated mfglib local repo?
[] Some example notebooks/pages on doc may still be helpful like here. Maybe can add more on basic usage, performance comparisons, tuning example, multi-Nash and even some variants. Also some fancy visualizations, etc.?
[] Add details on the implemented env's input parameters for easier lookup. Also add details on returns. Optionally better display of the API. Explain the usage and creation of initial policies using Policy and directly from PyTorch. Also API for non-leaf functions, like mean_field.py.
[] More details for contributing and developers.