Try out codeowners

I just learned about this feature. We can apparently default who to
delegate review to.

https://docs.github.com/en/github/creating-cloning-and-archiving-repositories/creating-a-repository-on-github/about-code-owners

> Code owners are automatically requested for review when someone opens a pull request that modifies code that they own.

Author: agjohnson

.github/CODEOWNERS

@@ -0,0 +1,4 @@
+*       @readthedocs/backend
+*.js    @readthedocs/frontend
+*.css   @readthedocs/frontend
+*.less  @readthedocs/frontend