[docs]: Boot stage picture

[blackboxsw]

Documentation request

Please provide a simple diagram/visual that shows boot stages -> related system units/services to give a quick overview of boot in https://cloudinit.readthedocs.io/en/latest/explanation/boot.html#boot-stages

This will aid new users in understanding what "boot stages" are and how they relate to systemd or boot services related to cloud-init.

Additional wants:

• when SSH or login is possible
• when network is configured
• when IMDS accessed
• horizontal diagram, simple and easy to glance at