Add more detailed docs

[eytanadler]

Description of feature

Add more detail to the docs. Some initial ideas are

• Beginners guide: introduction to building a simple aircraft model (discussing the necessary parts) and mission analysis
• Advanced guide: discuss introducing more complex propulsion models, thermal models (unsteady components), or mission profiles
• Tour of existing tools: describe the existing models that can be used for each discipline and where they can be found
• Developer's guide: what you need to know for developing your own models (for propulsion, hydrogen, aerodynamics, etc.)

[eytanadler]

Closed by #41