Change documentation about importing from extensions

[SychO9]

https://github.com/flarum/framework/issues/3707

A lot of extensions are also using the same method tags use flarum/extension. We should move to that format and enforce a convention of flarum/extensions/extension-id or smth.

This should wait for 2.0 when we have the export registry.

[askvortsov1]

Is export registry necessarily 2.0? Iirc it should be doable to introduce it without breaking changes, then deprecate the current import mechanisms, and remove those in 2.0.

[SychO9]

1.8 is the last 1.x release, we're just fixing bugs and making some performance improvements. The export registry seems too big of a task at this stage imo. I at least have yet to look into the details of it from 2021.