Docs: CI patterns for organic and external snaps

[medubelko]

What needs to get done

An unscheduled document was provided to us: https://forum.snapcraft.io/t/ci-patterns-for-organic-and-external-snaps/41963

We should:

• Assess its accuracy and suitability.
• Incorporate it into the official docs where possible.

Why it needs to get done

Public documentation contributions.

[sed-i]

Thanks @medubelko!
I believe this gap in docs is a substantial obstacle: I encounter it every time :)
I think documenting how to set up worry-free/fire-and-forget version bumps is key to publishing snaps.