Add a FAPI file upload example

Problem/Motivation

Looking for examples or documentation on how to correctly implement a file upload in a (settings) form in Drupal 8+ I saw it's not really documented and unclear in large parts, how to do it best-practice from start to the end.

I found some issues here, referencing AJAX examples, but I think that's also not showing an MVP example and is not the place where you'd look for that.

So this is a discussion on how and where to add an example for a file upload in a form.

Steps to reproduce

Proposed resolution

- Add an example to the Form Examples
- Add an API Docs page for file uploads or add examples to the FAPI element API docs

Remaining tasks

User interface changes

API changes

Data model changes