Improve documentation related to the OpenTelemetry Collector Builder

[mx-psi]

Per open-telemetry/opentelemetry-collector-releases/pull/164, we want to promote usage of the OpenTelemetry Collector Builder so that people build their own distribution with only the components they use.

The only documentation we have on the OpenTelemetry Collector Builder is its README, which has an example and some information about the configuration schema. We don't have any docs about this on https://opentelemetry.io (at least I couldn't find them!).

There are at least two fronts where I think we need to improve the documentation:

• [ ] open-telemetry/opentelemetry.io/issues/1555
• [ ] Document the replace directives that we need on OpenTelemetry Collector Contrib for the builder