Create a list of Pillow plugins in the documentation

[aclark4life]

I am ok without the new classifier. I would like to see a list of supported Pillow plugins somewhere in the documentation or readme file, so that people can find them more easily, if they need them.

Originally posted by @bigcat88 in #8892

I don't know if we can claim any of these are supported but we can list them in our documentation.

[radarhere]

See what you think of #8910 - https://pillow--8910.org.readthedocs.build/en/8910/handbook/third-party-plugins.html

[hugovk]

Oh, when @aclark4life self-assigned, I assumed he was going to work on it.

[radarhere]

Oh, apologies, missed that.

[aclark4life]

No worries! LGTM, thanks @radarhere . I can send some follow up PRs if needed.