grub.rst: Add info about showing the grub menu once and how to hide it in Linux Mint 21.3 and newer

[carlosmintfan]

Fixes: https://github.com/linuxmint/mint21.3-beta/issues/56 (but the repo is archived haha) Please check that everything I wrote is true.

[clefebvre]

No, the change in 21.3 didn't actually work so nothing is changed in this release.