Add SPDX 3.0 (new PR)

[bact]

• Add SPDX 3.0 page
• PLEASE DO NOT MERGE until PR Add SPDX 3.0 + Update SPDX 2.3 crosswalks codemeta#429 get merged and the crosswalk.csv get regenerated
• This PR is exactly the same as the merged PR Add SPDX 3.0 page #71 that get reverted by PR Revert "Add SPDX 3.0 page" #72

[meldra]

Hey @bact this is going to be affected by #130 when it is merged in the coming days.

There is no urgency to fixing it of course, since the crosswalk itself still is not merged, but just dropping this ping in here so you're aware.

[bact]

Thank you for heads-up.
I will recheck this PR again and make it align with the new structure/workflow.