Add SIF documentation

[SebastianPrehn]

This issue is created to track the development of the documentation for SIF.

As it stands, SIF will need the following:

• Its own section on the sidebar
• Sign up page
• How projects works
• What are participants, and how do they work

[SebastianPrehn]

@aputtu What would you determine is missing from the SIF documentation in our support system? Participants are somewhat covered in our project page, although it might benefit from further expansion.

[SebastianPrehn]

Seems to have all topics covered. Quality assurance phase.