Create guidelines for connector type owners to know how and when to flag user type errors

[mikecote]

Now that our metrics can distinguish between systematic errors and user errors, we should create guidelines for connector type owners to go through and flag user errors accordingly. Having guidelines will allow for consistent flagging across connector types and explain the importance of them.

Depends on: #180419

Definition of Done:

• Guidelines contains a breakdown of HTTP status codes that should be flagged as user errors
• Guidelines contains the definition of a user error and the level of accuracy required before defining errors as user errors
• Guidelines contains the context and importance of properly flagging user errors
• Guidelines contains code examples to flag and test the user errors
• Connector best practices updated with our guidelines and how-to for flagging user errors (https://docs.elastic.dev/reops/developer-guide/best-practices-connectors)

cc @kobelb @heespi

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)