Highlight OCB in the Collector docs

[julianocosta89]

The problem

The community started to release some opinionated Collector distros. While this is super helpful to give a head start to people using a collector on k8s (e.g). On the other hand, the otelcol-k8s distro lacks vendor specific components, what is understandable, but anyone using a collector in k8s would like to have cloud specific data, provided by receivers from AWS, Azure, GCP and so on.
To have that, they wouldn't use the opinionated distro, but would rather build their own.

Currently the custom-collector page gets lost in the middle of all docs.

The suggestion

I'd suggest highlighting the OCB more in the docs, provide more examples, and stress that this should be the default way to deploy the collector.
Unless you decide to use a vendor collector distro, Core is too basic, and Contrib is way too big, so we should improve the way we approach OCB.

Additional context: open-telemetry/opentelemetry-collector-releases#532

[svrnm]

Agreed, a few suggestions:

• Mention distributions & OCB already on the index page (https://opentelemetry.io/docs/collector/)
• Add a note at the top of the "Install the Collector" about OCB and distributions (https://opentelemetry.io/docs/collector/installation/), this page in particular recommends contrib at multiple places, so we might want to switch to core throughout the docs, or have subsections for different distros