DOC: document how to use installed data files with editable installs

[dnicolodi]

Replaces #786.

[rgommers]

Thanks @dnicolodi. The added text about how to use data files LGTM (modulo the minor open comments) and is useful.

It doesn't quite address the "can't use the NumPy C API from another package when NumPy itself is installed in editable mode". It can be added to this PR, probably as a .. warning:: note at the end, or left for a separate follow-up.

[dnicolodi]

It doesn't quite address the "can't use the NumPy C API from another package when NumPy itself is installed in editable mode". It can be added to this PR, probably as a .. warning:: note at the end, or left for a separate follow-up.

I feel that the numPy issue is too specific to be mentioned in the general documentation for editable installs. I think that the documentation now states clearly the reason why the headers cannot be used. Anything more specific would be more discoverable in the NumPy documentation or in a new "Papercuts" section of the documentation (maybe started by renaming the current "Limitations" section).

[rgommers]

Anything more specific would be more discoverable in the NumPy documentation or in a new "Papercuts" section of the documentation (maybe started by renaming the current "Limitations" section).

Fair enough. And I like the Papercuts name. I can have a look at it, also seeing if/where it needs a warning in the NumPy docs.

[rgommers]

I’d personally go that way yes. But don’t feel super strongly about it either

[dnicolodi]

I'll keep the note about the issue with importlibn.resources in Python 3.9 in the .. warning directive because I like it more there.

[rgommers]

I can have a look at it, also seeing if/where it needs a warning in the NumPy docs.

The NumPy docs at https://numpy.org/devdocs/building/index.html have this warning at the bottom of the section on editable installs, so that part should be covered:

[rgommers]

LGTM now, and the added test is also nice. In it goes.