[utilization] Update documentation to account for weekly projects

[kingcomma]

As the work towards #1500 proceeds, the documentation in utilization.md is becoming increasingly out of date. Update the doc to include:

• Description of how allocation to weekly billing projects is included in the utilization calculation
• Scenarios/examples for users tocking to weekly projects
• Definition of total_weekly_allocation value
• Updated definitions for target hours and utilization
• Description of how utilization works for users an alternative work schedule (AWS)
• Description of behavior for users with a 0% billable expectation (i.e., 18F Director)