Weight model documentation and write-up

[argonaut22]

Like for the other fields, we want to document the weight assumptions and provide a high-level summary for them (e.g., what scales with payload?, what's explicitly modelled? how are the different weight fractions normalized?). A placeholder page exists in the docs already

• generate summary of weight modelling approach
• integrate theory chunks and equations from Drela in "Theory 📖" blocks
• integrate weight breakdown diagram (Figure 1 in tasopt.pdf)