Add documentation for integration platform configuration

[astefanutti]

Integration platform CR is part of Camel K public API and as such should be documented.

I've seen a number of people asking for how to configure repositories, Camel version, ... Documentation of the platform CR should address their needs.

In addition, schema validation should be added to the CR.

[zregvart]

Noting a project that might help with adding the validation.

[astefanutti]

OpenAPI structural schema used for validation has been added by #1582.

For general purpose documentation, let's follow up in #1137.