Fix missing API interface section in documentation

.gitlab-ci.yml

docs/contact.md

docs/documentation/additional-software.md

docs/documentation/analysis.md

docs/documentation/libraries.md

docs/documentation/overview.md

docs/get-involved/testing.md

docs/index.md

mkdocs.yml

scripts/CSR-02.xml

scripts/copy_aircraft_design_to_docs.py

scripts/document_aircraft_xml.py