add first version of ClusterProvider documentation

[Diaphteiros]

What this PR does / why we need it:

Which issue(s) this PR fixes:
None

Special notes for your reviewer:

Release note:

Added a first version of the ClusterProvider documentation.

[ValentinGerlach]

Thank you for taking the time to write this down! I left some comments for things that we can improve in the future.

[ValentinGerlach]

I suggest that we split this predicate function into two:

Suggested change

	ctrlutils.HasLabelPredicate(clustersv1alpha1.ProviderLabel, providerName),
	// this just checks whether the label exists, independent from its value
	ctrlutils.HasLabelPredicate(clustersv1alpha1.ProfileLabel, ""),
	ctrlutils.HasLabelValuePredicate(clustersv1alpha1.ProviderLabel, providerName),
	// this just checks whether the label exists, independent from its value
	ctrlutils.HasLabelPredicate(clustersv1alpha1.ProfileLabel),

[Diaphteiros]

That is probably a good idea. Alternatively, we could also introduce a constant like ArbitraryValue that contains a string which is invalid as label value anyway. Both probably better than an empty string meaning "it doesn't matter".
Independent from how we improve this, I would still first change the implementation and update the documentation afterwards.

[ValentinGerlach]

This might be confusing for new collaborators. Can we simplify it to some thing like this?

Suggested change

	if err := ctrlutils.EnsureAnnotation(ctx, myClient, obj, apiconst.OperationAnnotation, "", true, ctrlutils.DELETE); err != nil {
	if err := ctrlutils.RemoveAnnotation(ctx, myClient, obj, apiconst.OperationAnnotation); err != nil {

[Diaphteiros]

Hm, the idea was that the EnsureAnnotation ensures a desired state of the annotation, either that it has a specific value or that it doesn't exist. But I have to admit that the deletion case doesn't look too nice syntactically.