Include generators on extensibility documentation

https://docs.conan.io/en/latest/extending.html

There's currently no reference or link on this page to custom generators as being an extension point. Seems like one of the most frequently used extension points, so should at least be listed here.

Open the linked extending.html page and review how it lists extension points. Add a reference or link identifying custom generators as an extension point, then confirm the page clearly points readers to the generator documentation.】【。

Written by the indexing model from the issue text.