[docs] Add LLM context files

### 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:
- ESLint fixer guide for LLMs 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
- Bundler setup instructions