Sections have been moved to:
* reference/fixtures.rst
* how-to/fixtures.rst
* fixtures.rst
according to their function. Further refinement and rewriting will be required.
Some material has been moved to a new "Anatomy of a test" document, in
anticipation that material from other sections will also find a natural
home there later.
Removed several unneeded reference targets from fixtures documentation.
Moved various documents into subdirectories, how-to and
reference.
Updated multiple links to use `:ref:` instead of `:doc:`,
meaning that files can henceforth be moved around without
breaking references.
Daniele Procida