Use crd-ref-docs for API reference documentation generation

similar to PR#563 for metal-operator this replaces outdated
[`github.com/ahmetb/gen-crd-api-reference-docs`](https://github.com/ahmetb/gen-crd-api-reference-docs)
(v0.3.0, last updated 2020) with
[`github.com/elastic/crd-ref-docs`](https://github.com/elastic/crd-ref-docs)
(v0.2.0).

also removes obsolete templates, changes config format to yaml.

Author: xkonni

Makefile

@@ -190,7 +190,7 @@ ENVTEST ?= $(LOCALBIN)/setup-envtest
 GOLANGCI_LINT = $(LOCALBIN)/golangci-lint
 ADDLICENSE ?= $(LOCALBIN)/addlicense
 GOIMPORTS ?= $(LOCALBIN)/goimports
-GEN_CRD_API_REFERENCE_DOCS ?= $(LOCALBIN)/gen-crd-api-reference-docs
+CRD_REF_DOCS ?= $(LOCALBIN)/crd-ref-docs
 KUBEBUILDER ?= $(LOCALBIN)/kubebuilder
 SETUP_ENVTEST_VERSION ?= v0.17.1   # example pre-0.22
 
@@ -204,7 +204,7 @@ ENVTEST_K8S_VERSION ?= $(shell go list -m -f "{{ .Version }}" k8s.io/api | awk -
 GOLANGCI_LINT_VERSION ?= v2.1
 ADDLICENSE_VERSION ?= v1.1.1
 GOIMPORTS_VERSION ?= v0.31.0
-GEN_CRD_API_REFERENCE_DOCS_VERSION ?= v0.3.0
+CRD_REF_DOCS_VERSION ?= v0.2.0
 KUBEBUILDER_VERSION ?= v4.5.1
 
 .PHONY: kustomize
@@ -256,13 +256,13 @@ $(GOIMPORTS): $(LOCALBIN)
 	$(call go-install-tool,$(GOIMPORTS),golang.org/x/tools/cmd/goimports,$(GOIMPORTS_VERSION))
 
 .PHONY: docs
-docs: gen-crd-api-reference-docs ## Run go generate to generate API reference documentation.
-	$(GEN_CRD_API_REFERENCE_DOCS) -api-dir ./api/v1alpha1 -config ./hack/api-reference/config.json -template-dir ./hack/api-reference/template -out-file ./docs/api-reference/api.md
+docs: crd-ref-docs ## Run go generate to generate API reference documentation.
+	$(CRD_REF_DOCS) --source-path=./api/v1alpha1 --config=./hack/api-reference/config.yaml --renderer=markdown --output-path=./docs/api-reference/api.md
 
-.PHONY: gen-crd-api-reference-docs
-gen-crd-api-reference-docs: $(GEN_CRD_API_REFERENCE_DOCS) ## Download gen-crd-api-reference-docs locally if necessary.
-$(GEN_CRD_API_REFERENCE_DOCS): $(LOCALBIN)
-	$(call go-install-tool,$(GEN_CRD_API_REFERENCE_DOCS),github.com/ahmetb/gen-crd-api-reference-docs,$(GEN_CRD_API_REFERENCE_DOCS_VERSION))
+.PHONY: crd-ref-docs
+crd-ref-docs: $(CRD_REF_DOCS) ## Download crd-ref-docs locally if necessary.
+$(CRD_REF_DOCS): $(LOCALBIN)
+	$(call go-install-tool,$(CRD_REF_DOCS),github.com/elastic/crd-ref-docs,$(CRD_REF_DOCS_VERSION))
 
 .PHONY: kubebuilder
 kubebuilder: $(KUBEBUILDER) ## Download kubebuilder locally if necessary.