Merge pull request #428 from pulp/mhrivnak-231-release-notes

adding 2.3.1 release notes
Update doc versions to 2.4.0.
Change the doc versions to 2.3.
Merge branch 'rbarlow-doc_versions'
Merge branch 'pulp-2.3'

Conflicts:
	docs/tech-reference/conf.py
	docs/user-guide/conf.py
Merge branch 'master' into distributed-tasks