Put the Cookbook contributors guide into the main site

[brian-rose]

As discussed at recent Pythia meetings, we want to elevate the Cookbook Contributors guide by styling it with the Pythia theme and making it easier to find.

[brian-rose]

Plan:

• [x] Move the markdown to this repo
• [x] Modify the prominent button on the gallery page to say "How can I make a new Cookbook?", and have it point to the guide
• [x] Put the "Submit a new Cookbook" button at the end of the Cookbook contributors guide instead of on the gallery page
• [x] Update all links to the Cookbook guide for internal consistency

[brian-rose]

All done!