Rename dir: spdx-3.0/ -> spdx3.0/ for consistency

[bact]

Follow naming convention used by examples in spdx2.*. These files and dir are renamed:

• spdx-3.0/ -> spdx3.0/
• example-13.* -> example13.*
• example-13-spdx-3.json - > example13.spdx3.json

Also update the new names inside:

• software/README.md and software/example13/README.md
  • Also rename the existing mentioned of spdx/ to spdx2.2
• the pull request workflow, as a follow up of #86, as agreed during the 2024-06-11 Tech team call.

And move example13's draw.io diagram to the root of the example, as it is not SPDX version-specific, also show the diagram in the example's README.

(This is partly a rework of #82)