[REQ] Add auto-generated doc to scala-akka client

Is your feature request related to a problem? Please describe.

• Add auto-generated doc (API, Model) to scala-akka client similar to what've done in other clients.

• improve the README (example)

Additional context

• Add doc path https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PowerShellExperimentalClientCodegen.java#L45-L46)

• Add doc template https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PowerShellExperimentalClientCodegen.java#L92-L93

• Customize the doc file name (https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PowerShellExperimentalClientCodegen.java#L613, https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PowerShellExperimentalClientCodegen.java#L628)

• Provide the doc templates, e.g. https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/Java/api_doc.mustache, https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/Java/model_doc.mustache