Follow reuse/spdx standards + license docs under CC-BY-4.0

[mara004]

See https://reuse.software and https://spdx.dev

Unless I missed something, the docs had no license information yet, so I am suggesting CC-By-4.0, but this could of course be changed to any other license of your preference.

There are some files where I'm not sure who created them, thus this MR does not make the repository fully reuse compliant yet. You can obtain a list of files that lack license/copyright data via reuse lint (requires the reuse package).

I think the spec actually requests an author email to be given, but I couldn't find your address. If you could tell me your email I'll add it.

(Side-note: In case PR #222 gets merged, the location of license_file would need to be changed accordingly.)