doc: update page for user supplied scripts

[inkisu]

Hey,

I appreciate your work on the project. While I was fiddling around with desktop notifications and the onevent / on_song_change_hook I had some issues finding the User-supplied-scripts.md and understanding the provided environment variables.

After writing a script, I decided to update the documentation and share my script.

Could this site be added to the https://docs.spotifyd.rs/ wiki?
Is it done by adding a link to the SUMMARY.md?

[eladyn]

Oh, thank you for reviving this page. I think it might be useful to many new users. I think that it might indeed be a good idea to add it to SUMMARY.md so that it can be discovered easily.