[#1380] Encourage installing latest release in source install docs

Needed an automated way to get the latest version number, so I added
functions to doc/conf.py for this, and then used them in the source and
package install docs.
[#1380] Remove hardcoded filename
[#1380] Fix some broken hyperlinks in the docs

Stop Sphinx from hyperlinking part of the URL in pip install commands.
[#1360] Update package_update docstring.
Merge branch '1380-encourage-installing-latest-release'
Merge branch '1360-update-docstring'

Conflicts:
	ckan/logic/action/update.py