Fix icon license data

[stefan6419846]

This adds the license text and proper source indicators for the used icons. To get a clear licensing situation, I switched to a properly released upstream distribution (which avoids having to deal with both the Apache-2.0 and OFL-1.1 license).

Corresponding discussion: #903

[pradyunsg]

Instead of having this appended to the license file, I'd prefer if we instead added a LICENSE.icons file and updated pyproject.toml to use license-files instead of license = { file = ... }.

I did just spend a bit too much time enabling sphinx-theme-builder to actually make it possible to do this change. ^.^

[stefan6419846]

I will have a look at this on Monday. My initial approach was to use a dedicated file, but it would not end up in the distributions during my testing. Thus I settled with the current approach.

[pradyunsg]

Yea, I needed to add support over in sphinx-theme-builder for it.

https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license-and-license-files is the relevant section -- it might make sense to just not have license at all since the files clearly reflect the relevant license.

[stefan6419846]

I have updated the files accordingly. Due to warnings, I have replaced the trove classifier with the PEP 639 approach for declaring the SPDX license expression accordingly, besides declaring the license files as specified in the same PEP.

[pradyunsg]

Did you also lookup license metadata for icons.html entries? It'd be good to have that too in the LICENSE.icons file.

x-ref #899, which is gonna involve making derivates of them 😅

[stefan6419846]

Did you also lookup license metadata for icons.html entries? It'd be good to have that too in the LICENSE.icons file.

I will try to take a detailed look on this next week and update the PR accordingly.

They mostly seem to be Tabler Icons (MIT) and Feather Icons (MIT) according to the class names (a short check revealed a match as well).

The only not really clear aspect is regarding the TOC icon changed in 2804674, but judging from a quick search of the drawing commands, it most likely is a modified copy of Ant Design Icons (MIT).