Properly support rebuilds

[chrisjsewell]

On running sphinx-build for a 2nd time, you currently get warnings:

WARNING: (broken) content-types/markdown.html redirects to file-types/markdown.html but /Users/chrisjsewell/Documents/GitHub/jupyter-book/docs/_build/html/content-types/markdown.html already exists!

Possible approaches:

1. keep track of written redirect files in the sphinx env
2. Add an "identifier" comment at the top of the redirect files, then check for it