For users of React < 19, there is a new react-compiler-runtime package
that can be used to provide a "polyfill" for runtime APIs needed for
compiled code.

This PR adds docs for that.

Co-authored-by: Sebastian "Sebbie" Silbermann <[email protected]>

Currently it's a little hidden, let's move it to its own subheading for
more prominence

Updates our compiler docs for the latest Beta release.

* [compiler] Remove section on healthcheck

This package will be deprecated soon. It makes adopting the compiler
confusing since it can spread the misconception that you need to have
all your components successfully compiled before you can use the
compiler.

For now let's remove this from our docs and deprecate the package on
npm. We can always choose to repurpose this in the future.

* Clarify that 100% compilation is not necessary

Add a new blog post announcing the React Compiler Beta release!

* Add ESLint flat config example, fix ESLint name

* Use official terminology

* Fix key