Reflect headlamp-plugin's exported modules in API docs

[joaquimrocha]

Description

Our docs lead to believe that plugins can import the ApiProxy functions by using @kinvolk/headlamp-plugin/lib/k8s/apiProxy but this doesn't work since we do expose the ApiProxy from @kinvolk/headlamp-plugin/lib itself.

Impact

Docs are confusing for plugin developers.

Expected behavior

The API docs reflect only the modules that can be correctly imported in plugins.