Update core calm concepts docs

[Budlee]

Feature Request

Description of Problem:

The Core calm concepts docs are out of date.
Since spending more time on the more recent CALM specs there are new fields that I have no clue what they mean or their intention.

A few lines should be added to any missing concepts and their meaning.

[Budlee]

@jpgough-ms and @rocketstack-matt, I have pushed a PR to update the Calm docs defining CALM. I mentioned to @jpgough-ms I have no idea what the instance value is: https://github.com/finos/architecture-as-code/blob/0.5.0/calm/draft/2025-01/meta/core.json#L65

It's completely unused and undocumented. I'd suggest gets removed from the Spec