docs for user management (+ general doc update)

With the merge of #25 (closed) it is now possible to add new users and modify some basic parameters for them. We need docs update to reflect those changes.

Other doc changes:

• Add pointer to stork.readthedocs.io in README.md
• Intro section (remove PDF, TXT)
• Stork backend API update
• Add a section about agent API
• How to deploy stork-agent