[docs]: add a minimum viable "getting started" section to the index

# Documentation request

It would really aid comprehension to provide a minimal usage example, e.g. perhaps something that creates a systemd service.

I envision including a basic `#cloud-config` YAML document + the bare minimum instructions for how it's expected that you get the file onto the box and how you make cloud-init execute it.

Otherwise the reader is left with a huge variety of disparate concepts and it's not clear how they all tie together.

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*Reported from: https://cloudinit.readthedocs.io/en/latest/index.html*