Revise and consolidate documentation

[lukpueh]

Description of issue or feature request:
In addition to #276, which deals with a technical overhaul of the documentation, we should also revise and consolidate in-toto documentation and supporting material which is scattered across multiple repos of the in-toto GitHub organization.

This issue is motivated by feedback from @ultrasaurus in in-toto/specification#15 (comment).

Current behavior:
in-toto documentation is available in various places, most notably:

• in-toto/in-toto (main README.md)
• in-toto/demo (main demo)
• in-toto/docs (specification)
• in-toto.io (website)

Expected behavior:
Revise up-to-dateness and suitedness of different docs for their respective purposes.

From in-toto/specification#15 (comment):

• add contextual information and a strong narrative around a typical use case
• spell out demo simple workflow (e.g. with numbered list: checkin code, run tests, release software artifact)
• re-think role names (Alice, Bob and Carl)