Improve the main documentation page

I spent some time re-arraging, re-naming and writing some production setup help under installation guide. The installation guide is more of help after you have installed mailman and need help configuring it or pointers to how things work.

http://docs.mailman3.org/en/new/index.html

It is live here at above url to see how it would look like. I would love some reviews, comments, concerns with this. A lot of hyperlinks are missing though, which I will update soon.

@terriko @abompard @warsaw @flux @msapiro It would awesome if you could have a look!