Give nested pages the right URLs

Put nested pages inside their parent folders, but redirect from the
top-level name to match WordPress using jekyll-redirect-from.
And, set permalink: pretty to reduce useless /index.md
Stop Kramdown from decoding HTML entities

Mostly, this just stops the smart quotes that WordPress put everywhere
from looking bad in development since WEBrick doesn't set a UTF-8
Content-Type header.
Add a layout for pages