Consistent language for "Contributors"

[brian-rose]

We currently have a mix of references to "Contributors Guide" and "Contributor's Guide" through our pages, and it would be good to clean things up for self-consistency.

Both are not quite grammatically correct, it should probably be "Contributors' Guide" -- but this starts to look a bit awkward.

Following some discussion with team, we prefer a more active language approach: "Contributing to Project Pythia", "Contributing to Foundations", "Contributing new Cookbooks", etc.

This is more in line with the language at Numpy, Xarray, Matplotlib, and other projects.

Addressing this will require some text edits across multiple repos.