Get started with composition

The goal of this guide is to:

* Introduce composition as a standalone concept (i.e. no MRs needed)
* Showcase a namespaced composition
* Showcase some different composition functions - i.e. different config language options

I'm also taking a different approach to the previous getting started guides, in that
I'm leaving out a lot of explanation of what things are. Instead I'm prioritizing having
folks apply things (even if they don't fully understand them) and seeing the results.
I feel more detailed explanations should come after the get started guides - e.g. in the
concepts section.

Signed-off-by: Nic Cope <[email protected]>

Author: negz