Automate release builds

[cardillan]

Three modifications to build scripts:

• Bumped the version of the checkout action in commitTest
• Added a Github action to automatically build the mod for published releases. I've tested this script on this mod template and on my other mod(s), but I don't understand all subtleties of Github actions (it was originally created with the help of AI). This needs to be reviewed for possible blunders.
• Added an icon.png file to the packaged jar. I've tested both local and Github builds to see it still works even when the file doesn't exist. I've seen mods that do include the icon in their root dir, and the mod browser shows it, but once the mod is installed, the icon isn't shown. This solves the issue. If this is not a good idea, please drop it.

Readme has been updated to make the users aware of the automated build.

If this is OK, I'll try to do the same for the Kotlin mod.