exclude PLForecastingModule docs from custom rnn and blockrnn modules

[dennisbader]

Summary

Adds custom .rst files for RNN and BlockRNNModules to avoid generating the docs from PLForecastingModule for Custom Modules.

Additional Information

• Makefile auto generates documentation for all modules using the templates defined in docs/templates
• Added a new folder docs/generated_api for custom .rst files for the generated API. All files in this folder will be copy pasted into the the auto-generated folder docs/source/generated_api.