feat(3d-synthezier): run city generator on Linux via crtbasic

feat(3d-synthezier): run city generator on Linux via crtbasic

- Add bin/city1.sh and bin/city2.sh, crtbasic equivalents of the DOS
  batch files: they run 3dparse.bas headless with --command selecting
  the scene (COMMAND$ stands in for the DOS /CMD switch).
- Change the include path in 3dparse.bas from the absolute DOS path
  C:\GRAPHICS\3D\3DSYNT~1\INCLUDE\ to the relative ..\include\,
  resolved against the bin directory, so no path editing is needed.
- Commit the generated city1.obj and city2.obj example scenes.
- Document the Linux workflow in a new "Linux (crtbasic)" section in
  doc/index.org.