Formally define and document the mixin spec

[rgeyer]

AFAIKT, there is no formal, documented "spec" for the format of a mixin.

It is implicitly defined by the eval methods, and discussed abstractly in these docs (which coincidentally should also be updated).

This is related to #36 which discusses the fact that typical jsonnet constructs are not natively supported (namely Top-level arguments and external variables).

In addition to supporting those (and all other?) jsonnet constructs, the mixin spec should be formalized and documented.

[gaantunes]

We are going to tackle a smaller scope for this quarter, which consists of documenting the practices we have implemented so far. This PR was opened for a review round: https://github.com/grafana/cloud-onboarding/pull/2711

[gaantunes]

Docs update PR: monitoring-mixins/docs#15