Updated naming of "getting-started.md" files for all TUB LB Python modules

...documentation/analysis/cost_estimation/getting_started.md

...ntation/analysis/cost_estimation/software_architecture.md

docs/documentation/sizing/fuselage_design/getting_started.md

...cumentation/sizing/landing_gear_design/getting_started.md

docs/documentation/sizing/tank_design/getting_started.md

mkdocs.yml