Add more to template landing page

[r-ford]

I think we should add more structure to the template landing page, based on the Radar Cookbook's landing page. Requiring at least motivation/background and structure/outline sections would give cookbooks some consistency.

[ktyle]

Currently, the README.md and the notebooks/landingpage.md in the Radar Cookbook are essentially the same, aside from the former's build-book and link-checker badges. Should we distinguish the content between these two files ... or perhaps trim down the README.md, replacing the descriptive text that is repeated in the landing page with a link to the landingpage (maybe with some "pithy" text as well)?

[r-ford]

That makes sense to me. I'm currently using the README.md as a place to put the Contributor's Guide, but I'll move it to its own file. In the guide, I can direct the contributor to replace the template's README.md with a title, the badges, a sentence or two about the cookbook (like it is in the Radar Cookbook before the Motivation section), and a link to the landing page.