Documentation improvements and refactoring

[csviri]

It's not easy to read / follow the current documentation. Would like to improve it, by continuously improving based on the following guidelines:

1. Would move sections like Patterns and Best Practices and Using sample Operators under Getting Started. These still would be separate pages, just under getting started.
2. Breaking down features page to sub-pages around topics. Like EventSources, Retries,Implementing Reconciler, and Higher level abstractions. And continuously improve these docs.
3. The page should always contains a simple introduction, and then basically a reference documentation that touches all the aspects and links also related integration tests.