chore(docs): migrate formal docs from Markdown to Typst

Replace .md formal documents with Typst sources and compiled PDFs:
- software_requirements_specification.typ/.pdf (was .md)
- software_design_specification.typ/.pdf (was .md)
- software_test_guide.typ/.pdf (was software_test_plan.md)

Renamed Software Test Plan to Software Test Guide (STG) to match
the canonical template naming convention.

Author: abitofhelp