[#1389] Tweak release process docs
Merge branch 'master' of github.com:okfn/ckan into 1389-release-docs-tweaks-2
Merge branch 'master' of github.com:okfn/ckan into 1389-release-docs-tweaks-2
[#1389] Reorganize the release process docs a little

Put all the stuff that users need to know (what the different types of
release are, and how to upgrade) in /upgrading.html, and put all the
stuff for developers only (how to do a CKAN release) in /contributing/.
Use ..  seealso:: boxes to link the two pages together. Also removed
some duplicated information, fixed some title styles, etc.
[#1389] Fix a Sphinx error
Merge branch '1389-release-docs-tweaks-2'