Restructuring the Nix manual

@edolstra a few notes below, there were some tasks that I wasn't able to do and some questions for you before you merge. 

To be completed before merging:
- Fix the build.nix script to pull files from sub-directories
- Build and test the manual locally (please copy the command and options to the comments so that I can transfer them to the Wiki page)
- Check that the restructuring of the command reference didn't break the man pages (if these topics are the ones generating the content)

Open questions:
- The "hacking" topic looks like it belongs in the quick-start part, so I didn't touch it - TBD
- There are a couple of files that I couldn't locate in the output or in the book file, so I left them as-is TBD
-- nix-lang-ref.xml
-- schemas.xml
- I moved the release notes away from the appendix but there was some xporter string that I wasn't sure about, so instead of deleting it I commented it out so you can look at it