Collocate docs with source code

[alirezamirian]

Should be done after #62.

Each component has an mdx file which is now living under docs folder in website package. They should each be moved next to the source code for that component, so code, tests, examples, and documentation are all collocated for each component.
The rest of the docs should be moved into a docs folder next to src.

• Update component template accordingly.
• Move component mdx files from website/docs/components to the component folder where source, tests, and examples are. Some internal linking needs to be updated too.
• Add flexibleSidebarItemGenerator to generate "components" sidebar based on category metadata in docs, rather than their path. (flexibleSidebarItemGenerator is planned to be published as a separate package).
• Include other docs from a docs folder, either in website, or in the library, next to src folder.