Unify getting-started & add libraries

...documentation/analysis/cost_estimation/getting-started.md

docs/documentation/analysis/cost_estimation/index.md

...ntation/analysis/cost_estimation/operating_cost_method.md

...nalysis/cost_estimation/run_your_first_cost_estimation.md

...on/analysis/weight_and_balance_analysis/basic-concepts.md

...cumentation/analysis/weight_and_balance_analysis/index.md

...cumentation/analysis/weight_and_balance_analysis/usage.md

docs/documentation/sizing/fuselage_design/getting-started.md

docs/documentation/sizing/initial_sizing/getting-started.md

...cumentation/sizing/landing_gear_design/getting-started.md

...documentation/sizing/propulsion_design/getting-started.md

docs/documentation/sizing/systems_design/getting-started.md

docs/documentation/sizing/tank_design/getting-started.md

docs/documentation/sizing/wing_design/getting-started.md

mkdocs.yml