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

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

Description

I noticed that the link to the "Getting started" pages for all TUB LB Python modules was broke due to a somehow changed naming of the linked file. To keep the naming of the documentation files consistent, I changed the naming of all TUB LB Python module "getting-started.md" files to "getting_started.md". Furthermore, I updated the paths in the mkdocs.yml file accordingly. This affects the following modules: fuselage_design, tank_design, landing_gear_design, cost_estimation

Furthermore, the software_architecture.md file for the cost_estimation tool was missing. I added the path to this file and will add the file with the cost_estimation documentation update.

Related Issue(s)

None

Other Changes

None

Screenshots/Logs

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Testing Instructions

1. Check all links to the "Getting started" page on the Index pages for the four modules
2. The link should work now

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.

Additional Notes

None