Add example for validating internal telemetry connection with `certificate` field

[tiffany76]

What needs to be changed? A request was made to document how to validate the connection for internal telemetry collection using the certificate configuration field.

What is the name + path of the page that needs changed? https://opentelemetry.io/docs/collector/internal-telemetry/#activate-internal-telemetry-in-the-collector

Additional context: Originally raised in https://github.com/open-telemetry/opentelemetry-collector/issues/13337

cc: @vistralmystral