[DOCS] Automate output for 4 case APIs (elastic#142173)

docs/api-generated/README.md

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+# OpenAPI (Experimental)
+
+Open API specifications (OAS) exist in JSON or YAML format for some Kibana features,
+though they are experimental and may be incomplete or change later.
+
+A preview of the API specifications can be added to the Kibana Guide by using
+the following process:
+
+. Install [OpenAPI Generator](https://openapi-generator.tech/docs/installation),
+or a similar tool that can generate HTML output from OAS.
+
+. Optionally validate the specifications by using the commands listed in the appropriate readmes.
+
+. Generate HTML output. For example:
+
+  ```
+  openapi-generator-cli generate -g html -i ~/kibana/x-pack/plugins/cases/docs/openapi/entrypoint.yaml -o ~/kibana/docs/api-generated/cases -t ~/kibana/docs/api-generated/template
+
+  openapi-generator-cli generate -g html -i ~/kibana/x-pack/plugins/ml/common/openapi/ml_apis_v3.yaml -o ~/kibana/docs/api-generated/machine-learning -t ~/kibana/docs/api-generated/template
+  ```
+
+. Rename the output files. For example:
+  ```
+  mv ~/kibana/docs/api-generated/cases/index.html case-apis-passthru.asciidoc
+  mv ~/kibana/docs/api-generated/machine-learning/index.html ml-apis-passthru.adoc
+  ```
+
+. If you're creating a new set of API output, you will need to have a page that incorporates the output by using passthrough blocks. For more information, refer to [Asciidoctor docs](https://docs.asciidoctor.org/asciidoc/latest/pass/pass-block/)
+
+. Verify the output by building the Kibana documentation. At this time, the output is added as a technical preview in the appendix.
+
+## Known issues
+
+- Some OAS 3.0 features such as `anyOf`, `oneOf`, and `allOf` might not display properly in the preview. These are on the [Short-term roadmap](https://openapi-generator.tech/docs/roadmap/) at this time.
+
+