[Dataset quality] Introducing Degraded docs rule

[yngrdyn]

In elastic/kibana#179173 we are introducing a new rule for data set quality page: Degraded docs rule.

The rule configuration contains mainly a dataStream name, a threshold value and a threshold comparator.
Degraded docs rule is a stack rule but it's visible from observability context (stateful and serverless).

We need to document this new rule for versions 8.19, 9.1 and serverless.

[mdbirnstiehl]

Thanks for making this @yngrdyn! I've updated the issue to go to the experience docs board and I'm going to remove myself as assignee for now, and we'll update the assignee as we actually start working on the issue.