allow projects to not have index.md

[Fil]

"docs/index.md is the home page for your site. You can have as many additional pages as you’d like, but you should always have a home page, too."

I feel that this requirement is a bit burdensome, especially for one-page projects which all end up with a docs/index.md file instead of more meaningful file names. Doesn't make it easy finding them in the Finder, etc. I don't have a fully formed suggestion yet but wanted to note it in passing. Maybe instead of serving 404 when index.md is missing, we could do something nicer, like a default splash page with a summary/index of subpages?