[docs/collector/scaling, docs/collector/deployment] Describe how to deal with single-writer principle

[mx-psi]

What needs to be changed?

The OpenTelemetry metrics data model requires each metric data stream to have a single writer. This spec section is exhaustive but it is written in spec-speak and it can be a bit hard to understand (and, possibly, to find).

Specifically in the Collector, it can be hard to understand that this is a requirement to avoid overwriting the same time series. Since we already have a couple pages dedicated to how to deploy the Collector, we can explicitly mention taking care of this in the documentation.

What is the name + path of the page that needs changed? Either of collector/scaling or collector/deployment or one of its subpages would be good for this.

Additional context: Example related issue open-telemetry/opentelemetry-collector-contrib/issues/32043