Document dashboards-as-code details

[jpetey75]

stemmed from this internal thread, but i have had people ask about this feature on demos too and i am not super familiar, so some more thorough docs would be super helpful

https://lightdash.slack.com/archives/C03MCQ08UKS/p1740436448204869

[jpetey75]

I think you needed some info from Javi when he's back, right @oli-rmsy ? I'll put in blocked for now for that reason.