Rework OpAMP visualization

[svrnm]

The Management page shows an image that demonstrates how OpAMP works. It would be great if that image (https://opentelemetry.io/docs/collector/img/opamp.svg), would look similar to other images in the documentation, e.g. https://opentelemetry.io/docs/collector/img/otel-gateway-sdk.svg

Special Note for Outreachy Contributors: if you want to work on this issue, please make sure that you have completed the small steps and at least one assigned issue as outlined here: #5290