Refactor (fold) the documentation

[GordonYuanyc]

• Too many contents in documentation
• Top of the README should be a list of supported sketches
• Quick start under second page
• other details can be redirected to other locations

[milindsrivastava1997]

I would say that quickstart is more important the list of supported sketches.
Quickstart -> headline performance result -> supported sketches.

Can we also add some info on why people should use this and not datasketches?