it would really be nice if there was some HOW-TO or documentation on all
this new UEFI/shim packages...

There is.

https://fedoraproject.org/wiki/GRUB_2

grub2-mkconfig will add entries for other operating systems it can find. That will be done based on the output of the os-prober tool.

That might however not work so well, especially not for booting other Linux operating systems, and especially not on UEFI systems

-- 
Paul Cartwright
Registered Linux User #367800 and new counter #561587