Add snippet to central config and optional step to EDOT quickstarts

[theletterf]

From @hegerchr:

For all deployment types with and without Docker. The snippet we have for activating apmconfig in the Collector should include this. The service section in the Collector config „activates“ it. The extension section configures the extension only.

Considering the example in central config docs something like this:

extensions:
  bearertokenauth:
    scheme: "APIKey"
    token: "<ENCODED_ELASTICSEARCH_APIKEY>"

  apmconfig:
    opamp:
      protocols:
        http:
          # Default is localhost:4320
          # endpoint: "<CUSTOM_OPAMP_ENDPOINT>"
    source:
      elasticsearch:
        endpoint: "<ELASTICSEARCH_ENDPOINT>"
        auth:
          authenticator: bearertokenauth

service:
  extensions: [bearertokenauth, apmconfig]

A suggestion I have is to add an info about this part

  apmconfig:
    opamp:
      protocols:
        http:
          # Default is localhost:4320
          # endpoint: "<CUSTOM_OPAMP_ENDPOINT>"
of the example too. All other examples (e.g. [here](https://github.com/elastic/opentelemetry-collector-components/tree/main/extension/apmconfigextension#getting-started)) I have seen use
  apmconfig:
    opamp:
      protocols:
        http:
          endpoint: "localhost:4320"

if it's only necessary to have endpoint: "localhost:4320" written in the config if you want something different than localhost:4320 and otherwise it is just fine to only have http: as the docs read, I think we should mention this.

Actions:

• Edit the snippet in https://www.elastic.co/docs/reference/opentelemetry/central-configuration to add the services part
• Add an (Optional) step in all quickstarts to enable Central config management, after the SDK step (also optional)