Add documentation describing which interfaces are supported.

[jaraco]

Closes #4978

[abravalheri]

Thank you very much for working on this Jason.

I have started drafting something to integrate with your text:

https://github.com/pypa/setuptools/compare/debt/supported-interfaces...abravalheri:setuptools:supported-interfaces-suggestions?expand=1

Please feel free to have a look and borrow whatever you want from the draft (of if you don't agree just feel free to ignore).

I can see that are a couple more related pages in the docs like https://setuptools.pypa.io/en/latest/userguide/extension.html and https://setuptools.pypa.io/en/latest/deprecated/index.html. So later we may try to coordinate the content among them.

[abravalheri]

Maybe we include that after quickstart?

I think I understand the need of this to be prominent, but we have 2 classes of users: "someone that wants to do something quick and simple" and "someone that wants to do something complex that requires customisation". Adding the new file before quick start may scare out the first time of users.

[jaraco]

Oops. I merged it but forgot to make this change. I've amended that in 14870dd.

[jaraco]

I have started drafting something to integrate with your text:

https://github.com/pypa/setuptools/compare/debt/supported-interfaces...abravalheri:setuptools:supported-interfaces-suggestions?expand=1

I've incorporated these suggestions wholesale. Thanks for adding your thoughts.