Add documentation for systemd units architecture

**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.

Start by reading the architecture refactor in bottlerocket-os/bottlerocket#1423 and comparing it with the current systemd unit organization in this website's content. Document the new architecture, explain why it differs from other distributions, and make the resulting explanation easy to find and verify against the refactor.

Written by the indexing model from the issue text.