[SLO] Update docs to highlight that burn rate rule is automatically enabled now

[paulb-elastic]

Description

Up until elastic/kibana#176317 was merged, users were given the option to create a burn rate rule, as they configured an SLO. This is no longer the case, and a default burn rate rule is now automatically created for each new SLO.

The burn rate rule will use the default configuration and no connector.

The docs here and here will need updating to reflect the new behaviour.

I would suggest we also make it clear that the default rule has no connector, so the user must configure this if they want to receive alerts for SLO breaches.

Resources

The change in behaviour was implemented in elastic/kibana#176317

Which documentation set does this change impact?

Stateful and Serverless

Feature differences

Both stateful and serverless

What release is this request related to?

8.13

Collaboration model

The documentation team

Point of contact.

Main contact: @simianhacker

Stakeholders:

• @grabowskit
• @paulb-elastic