Unify currently fragmented documentation

[MatthewDaggitt]

While trying to get a better understanding of the code base, I've noticed the documentation for Marabou is very fragmented. Places where I've found documentation:

• README.md
• Marabou Wiki
• ReadTheDocs
• A random notebook

There's a lot of duplicated and out-dated information scattered around. I think it would make a lot of sense to unify everything under the ReadTheDocs documentation, leaving only a minimal README.md on the front page.