Restructure documentation

[HoBeZwe]

Hi,

I've set up a documentation for BEAST in my fork as a suggestion. Not sure it aligns with your ideas/plans. Maybe, you can have a look at it here and see whether you are interested in a pull request. I've also adapted the README a bit.

The ideas of my suggestion:

• Separate the manual and internal details to some extent (large parts of the current documentation are now internals).
• Give more details on the actually implemented operators, bases, and excitations, ... like for the single layer including overviews such as here.
• Focus more on the assemble routines than on the @discretize macro.
• Make the examples part ot the docs: they are actually executed, hence it is always clear that they still work + some explanations can be added
• Logo: having one is probably nice but it is just some idea of mine. If you think it is not suitable, I could also remove it.

Of course, a lot is still missing, but it could serve as a starting point.

I would be happy to hear your thoughts on this.