Updates to systems documentation

Update systems documentation

Description

Adds info on minimal required input and why there is no mass scaling factor for the furnishing system.

Related Issue(s)

• Resolves aircraft-design#143 (closed)

Developer Checklist

• Code has been tested locally and/or in pipeline.
• (if applicable) documentation updated.
• (if applicable) impact of new dependencies reviewed and included in project.
• Merge conflicts resolved with the target branch.