Documentation for self-gravity and gravitational instability

[crislong]

New file "selfgravity_gravitationalinstability.rst". Documentation for simulate self-gravitating and gravitationally unstable discs (setups sgdisc, dustysgdisc, isosgdisc, dustyisosgdisc).

[danieljprice]

Thanks Cristiano! Suggest to add [skip ci] when committing only documentation changes then github doesn't try to run the entire build/test suite. We might also want to link this page from the table of contents somewhere (e.g. examples page)

[crislong]

Thanks for the suggestion Daniel! Yes, it could be nice, I will work on this tomorrow!