Guide to writing a composition

### What's Missing?

We have getting started guides but they just say "use this composition". We also have composition docs, but they're more of a reference and explaining the general concepts. It'd be neat to have a page walking you though writing your first composition.

Ideally without picking any one language. Maybe a lang selector to let you pick one of a few examples ala the other getting started guides.

This could help with how to think about compositions in general - e.g. the fact that you're really writing a chunk of a reconcile loop.