Extend documentation to include info on (national) modeling

The documentation of the model should be extended to add some information shared in recent collaborations with national modelling teams.
Items to include:
- How to get started working with MESSAGEix and setting up a national model.
- Various strategies on how to migrate work from an existing model framework to MESSAGEix.

Some information already collected can be found [here](https://iiasahub.sharepoint.com/:w:/s/ene/MESSAGEix/ES4tbDKPxYFNkz58pspZxmUBYvR3ixbsKZznYMU9l7NEzg?e=xgH5xt).