Add documentation for systemd units architecture

[arnaldo2792]

What I'd like:
In bottlerocket-os/bottlerocket#1423 we did a major refactor of the systemd units' architecture, and it will be great to have documentation explaining the new architecture and why is it so different compared to what other distros do.

[stmcginnis]

@arnaldo2792 do you know what else we need besides what was done in bottlerocket-os/bottlerocket#2735? Or can this be closed?

[arnaldo2792]

What I wanted out of this is a document that explains the entire boot sequence, what each unit does in early boot, how bootstrap containers units are generated and the units we use to configure the partitions.