[#769] Huge docs reorganization

The main changes are:

- Completely redesigned the main table of contents. This is intended to
  be a much simpler, clearer, more linear structure, both for readers to
  find their way and for developers to know where to add new docs.

- New "Installing" page that links to all the pages about
  installing/deploying/upgrading package installs and source installs.
  This is needed mainly so we have somewhere to explain the choice
  between package and source install.

- Move "Upgrading a Package Install to CKAN 2.0" and "Upgrading a Source
  Install" onto their own pages. These should be merged into a single
  "Upgrading CKAN" page in the future.

- Rename the "Deployment" page to "Deploying a Source Install", you
  don't need to do it for package installs.

- Renamed the "Post-Installation Setup" page to "Getting Started", moved some
  stuff from the "Theming" page to here, edited the page, adding a
  screenshot. This page could probably be improved a lot.

- Rename the "CKAN Paster Commands" page to "Command Line Interface" and
  rewrite it. It should be much clearer about how to use and
  troubleshoot paster commands now.

  Also move the paster commands reference into the same page, so all CLI
  docs are on one page.

  Moved this page much higher up in the table of contents so people
  should find out shortly after doing either a package or source
  install, how to run paster commands.

- Deleted the "Database Dumps" page, this was double-documented in the Paster
  Commands Reference, I merged the differences in.

- Deleted the "Extensions" page. We now just have a "Features" section
  that lists every core feature and core extension (each has its own
  page).

  Users don't care whether something is a core feature or a core
  extension, they just want to know what it does and how to use it.

  Writing extensions is still covered on the "Writing Extensions" page.

- Moved all the FileStore stuff (overview, setup, API) onto one
  FileStore page.

- Moved all the datastore docs: overview, setup, api onto one page.
  Moved datastore API reference docs from Action API page into datastore
  page.

- Delete the "Importing Datasets" page. Move the API example from this page onto
  the API page.

- Added module-level docstrings to the files in ckan/logic/action/*.py,
  for the benefit of the action API reference docs

- Put the Action API reference docs back onto the same page with the API
  overview docs, instead of splitting it all across 5 pages.

- Rewrote the CONTRIBUTING file, it now contains everything: reporting
  issues, translating, all the different coding standards, commit
  messages, frontend development guidelines, writing documentation,
  making and merging pull requests ... all in one place.

- Renamed CKAN Code Architecture to CKAN Coding Standards

- Renamed the "Testing for Developers" page to "Testing CKAN"

- Moved the "Frontend Testing" page into the "Testing CKAN" page, so now
  all the docs about testing are on one page.

- Deleted the "Common Error Messages" page. I moved some of the error messages
  to other places, the rest of them looked no longer relevant.

- Deleted the "Publishing Datasets" page. Just deleted it.

- Lots of editing all over the place, but much more is needed.
[#769] Change docs sidebar contents

- Show Next Page and Prev Page links in sidebar

- Don't show the current page's table of contents in the sidebar,
  this is already contained (and expanded) within the global table of contents
  that's already shown in the sidebar

- Don't show the table of contents in the sidebar, on the table of contents
  page!
[#769] Collect all release docs onto one page

Collect the release types, release cycle, release process etc. and
changelog together into one releases page. Also edited the text on this
page.
[#769] Move command line interface docs into features section
[#769] Move config options ref further down in docs
Merge branch '769-docs-reorg'

Conflicts:
	doc/datastore-api.rst