Reorganize README and categorize generators by use case

Our README currently displays all the generators in
collapsible lists, organized by their namespaces.

This makes it harder to find which generators to
use based on what I need.

This commit:

- shortens the README with links to appropriate
docs, and highlights the features we provide
- categorizes generators by use cases,
for example: Finance, Person, Number, Location,
Date, etc. I used faker-js as a reference.
- creates a new generators doc listing all the generators
by categories and namespaces

Author: stefannibrasil