add example for installing FPF plugin components that aren't published to NPM

I had a conversation with Felipe where he asked:

Do you know of any tutor plugin that loads the code of a UIslot without requiring that the code is first published to NPM?

I didn't know any examples. The README examples for the slots all use inline RenderWidget code, and the other examples use NPM packages.

The solution used for openedx-ai-extensions ended up being Tutor's build-contexts.

https://github.com/openedx/openedx-ai-extensions/blob/main/tutor/openedx_ai_extensions/plugin.py#L32

I definitely think "make a plugin that utilizes FPF slots but doesn't have frontend code published to NPM" is a reasonable use case that we should (and apparently already do) support, so it'd be great to document that.

Inizia con gli esempi di slot nel README e con l’approccio build-contexts a cui si fa riferimento in openedx-ai-extensions/tutor/openedx_ai_extensions/plugin.py at line 32. Documenta un esempio che mostri come usare gli slot FPF quando il codice frontend non è pubblicato su NPM e assicurati che l’esempio rifletta la soluzione build-contexts esistente.