Document using setup.cfg for src-layouts

[madig]

Summary of changes

Document how to use setup.cfg with src/ layouts.

Closes #1524.

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d.

[pganssle]

Just a few minor tweaks to the wording, overall this looks very good.

Also, one of these you use :: for a colon and in another line you use :. I am not super familiar with RST, is there a difference?

[pganssle]

Suggested change

	Using a ``src/`` layout (storing packages in a subdirectory)
	Using a ``src/`` layout

I think maybe we don't need the parenthetical part of this.

[pganssle]

Suggested change

	If you use a ``src/`` layout, as in, you package structure looks like this::
	One commonly used package configuration has all the module source code in a subdirectory (often called the `src/` layout), like this::

[pganssle]

Suggested change

	You can set up your ``setup.cfg`` to automatically look up all your packages in
	You can set up your ``setup.cfg`` to automatically find all your packages in

[pganssle]

Also, I'm happy to merge with just the documentation, but if you want to it would be nice to have a test that this setup.cfg configuration works properly.

[madig]

Oh right, I can put a test in somewhere.

Not sure about the double-colon, I just copy-pasted it from further up.

Edit: apparently needed to make a block.

[pganssle]

Ah, I see, the second one has the code block thing.

[jaraco]

Nice work @madig. Thanks for the review @pganssle.

[madig]

I can come up with a test when I get to it :)