docs cleanup

[joernhees]

it would be a good idea to deprecate / remove / update old docs. This is a meta-issue to collect out-of date stuff. Feel free to report via comments, or edit (if you can) the following list:

• http://rdfextras.readthedocs.io/en/latest/index.html
  • make sure examples are integrated in core docs
  • deprecate / remove
• main readme on https://github.com/RDFLib/rdflib
  • explain RDFLib eco-system: relations between all the projects, and their "state"
• main docs http://rdflib.readthedocs.io/en/stable/
  • improve mainpage (better overview, actual content)
  • maybe a more complete tutorial (walk through every major aspect?)
  • make sure stuff is up-to-date (easy install vs pip etc)
  • go through examples again

[iddan]

Maybe Dataset be encouraged as the entry object for data?