Document salt-wrapper

Summary:
The documentation is divided in two parts:

• an admin manual to cover installation and configuration
• an user manual to explain how to use and troubleshoot it

A contributor part could be added in the future,
especially if we prepare a rst generic document
explaining how to contribute to Nasqueron projects.

Test Plan:

• cd docs && make html
• proofread the generated documentation

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1024