Add provenance docs

[elsapassaro]

added provenance documentation file and figures

[giovannipizzi]

Hi Elsa,
yesterday after dinner Sebastiaan checked and improved the google docs text... sorry about this (see e.g. the beginning - or you can check the changes in the history).
Could you put the updated text?

[elsapassaro]

Ok done.

[ltalirz]

@asle85 Thanks for your work!

Just briefly - what I originally intended as the beginning of the "Concepts" section is under the title "CONCEPTS - Provenance" on p.15 of the google doc.
The basic idea is to start from a small data provenance graph, tell its story, and then introduce workflows by demonstrating that this becomes unmanageable for a larger graph.

I still think this is the most effective way to introduce new users to the concept, but we should merge your PR now, and I can work on merging this intro into your docs.

[coveralls]

Coverage decreased (-0.03%) to 68.979% when pulling c306854 on asle85:add_provenance_docs into 85d7309 on aiidateam:provenance_redesign.