Update OS installation instructions from merged iso testing expectations

[tlaurion]

Some random notes.

Ubuntu installs from love with expected unencrypted /boot and encrypted rootfs.

Debian dvd permits that too, but not live isos. Dvd required to be flashed on thumb drive with dd.

Review all linuxboot/heads expectations from testing and update docs comprehensively.

[tlaurion]

For example, to install NixOS to be compatible for Heads from linuxboot/heads#2155

Notes on NixOS installer custom partitioning requirements

- 2048M ext4 unencrypted, specify /boot mountpoint and grub-bios ticked
- 4096M swap, tick encrypted, put passphrase that will be same then rootfs
- Rest of unpartition space ext4, specify / mountpoint, tick encrypted, put passphrase that will be same then swap

When setting TPM Disk Unlock Key (DUK) on default boot entry to decrypt filesystems after kexec call, Heads prompts which if not all of LUKS containers are to have an additional TPM DUK added in nvram sealed with measurements+passphrase, and adds TPM DUK to prior defined containers that must have the same passphrase.

TLDR: NixOS installer, if dd'ed to seperate Thumb drive (issue to be opened on their side so that ISO is bootable (contain logic to find back UUID of where the iso is, not just support iso9660 which makes ISO boot unsupported, same as for Debian DVD and Ubuntu DVD, where livecd works but installer don't support unecrypted /boot easy end user setup: only DVD supports it. I know, its a mess but I can't do everything, help me out.

[tlaurion]

PR linuxboot/heads#2130 added https://github.com/linuxboot/heads/blob/master/doc/iso_boot.md (developer doc)

This needs to translate into user docs under https://osresearch.net/Install-and-Configure, so a pr to this repo. Adding help needed tag.