Update or remove hierarchy of methods overview in RST docs

[MichaelClerx]

Hi @ben18785 !

We have a hierarchy of methods in the PINTS docs, here: https://github.com/pints-team/pints/blob/master/docs/source/index.rst

I think this was a forerunner of your bigger & better interactive tool and the graphical overviews for the new paper. However, it's gone out of date a bit, and the bullet-point list isn't quite as versatile as your later efforts.

However, I do still think it's a useful way to present our index page, so maybe you could have a go at updating it?

Up to you! Happy to discuss / help where I can