With #506, the documentation files are listed as being provided
under the CC-BY-4.0 license, with the REUSE.toml mechansim.
Apply that to the top-level LICENSE file, making both mechanisms
consistent.

Use the CC-BY-4.0 text from https://creativecommons.org/licenses/by/4.0/legalcode.txt
which has reasonable line breaks.

Update LICENSES/CC-BY-4.0 to the same version of the text.

Fixed: #522