Chapter "Firmware Updates":
* improve the structure and clarity of information provided
* mention which update methods are when available/recommended
* add information about the already pre-installed pve-firmware package
* emphasise the importance of CPU microcode updates, how to interpret
versions and how to recover a possibly unbootable system
* move info about non-free-firmware repo to "Package Repositories"
Chapter "Package Repositories":
* add new section "Debian Firmware Repository"
Signed-off-by: Alexander Zeidler <a.zeidler@proxmox.com>