chore(makefile): simplify api-docs generation

[weisdd]

In #1506, we've discovered that kustomization.yaml in config/ did not contain a reference to grafana.integreatly.org_grafanacontactpoints.yaml, thus the resource was not included in our generated documentation.
Essentially, that means that whenever we introduce new resources, we need not to forget to update kustomization.yaml. - In the current flow, we use kustomize to build a big yaml-file and then feed it to crdoc.

I suggest to have a bit simpler flow:
crdoc is smart enough to read all files from a folder, but, unfortunately, it fails with errors if a folder has any other files or nested folders.
To workaround that, we can simply have a separate folder that would only contain full CRDs and then point crdoc to that folder.

[NissesSenap]

LGTM