[docs] Add context files for style authoring

[mellyeliu]

Describe the feature request

It'd be useful to have context files that document StyleX best practices for developers authoring styles with StyleX. This would include things like:

• Documentation on ESLint rules and instructions to disallow certain CSS properties or practices (border shorthands, disalllowed properties within StyleX, prefer conditional styles over dynamic styles when possible, etc)
• Documentation on APIs and API usage

[krishachikka]

Hi @mellyeliu ! I’d like to work on the context files. Can I take this on?

[mellyeliu]

@krishachikka Sure, feel free to take a stab at it! We'll probably need a few iterations, but we can start with some styling context. https://stylexjs.com/docs/learn/styling-ui/defining-styles/ will be a good reference.