Big docs reorganization

This is not as big a change as it looks like, because it's taking the way the
docs were already organized by audience and making it explicit by creating
top-level dirs for each audience and moving all the docs into them.
Also cleans up a couple of loose ends where particular files didn't fit into
the organization-based scheme, fit them in. It then has to touch a lot of files
in doc/ to update Sphinx cross-references.

Create 8 top-level directories, one for each different audience:
User guide, sysadmin guide, maintainer's guide, API guide, extending guide,
theming guide, contributing guide and changelog.
(User guide, sysadmin guide and changelog are actually just files, not
directories, because they only need one file each.)

- New overview on docs front page explains the high-level structure

- Added a section to the documentation guidelines explaining the new structure

- Lots of feature pages moved into new maintaining/ dir

- Configuration.rst moved into maintaining/

- getting-started.rst moved into maintaining/

- Move Solr multicore appendix page into maintaining/,
  remove the appendix section itself as no longer needed

- Remove the "Features" index page, no longer needed as the pages just go into
  the "Maintainer's guide" now

- Create doc/maintaining/installing/ subdir and move the installing and
  deployment docs into it

- Create doc/maintaining/upgrading/ subdir and move the upgrading docs into
  it

- Move api.rst and legacy-api.rst into new api/ dir

- Move test.rst into contributing/

- Fix Sphinx references all over the place to point to the new file locations
Merge branch 'master' of github.com:okfn/ckan into docs-reorg
Docs front page text tweaks
Merge branch 'docs-reorg'

Conflicts:
	doc/extensions/index.rst
	doc/features.rst

[ci skip]