Adapt physical server installation documentation

Notably, this:

• fixes how to enable the ilo remote access (the current described way was not the right one)
• agnostifies a bit some samples (so one can copy paste more easily the instructions)
• adds hint on how to exit the serial console (so one does not have to look for it)
• mentions the need to store the root password in the credentials store (explicit is better than implicit)
• Add one missing build dependency
• wraps lines to 88 or so

Refs. swh/infra/sysadm-environment#4808 (closed)

cc @teams/sysadmin