Simplify and Document how to bring your own runtime

[lresende]

Describe the issue
With the Elyra user base growing, we are seeing more folks asking for the ability to bring your own runtime and we need to simplify and document the requirements. See more details on the Bring your own runtime discussion.

Things to look at:

• Add the ability to bring your own schema #2075
• Implement a data-driven front-end that discovers and adapts to installed runtimes The new 'Airflow Pipeline Editor' feature is not working properly. DAGs created with this function will be recognized as errors by 'Airflow'. #2124
• Refactor runtime specific into its own source directories to make it more clear
• Add the ability to install individual runtimes separately #2089

Related discussion items:

• Bring your own runtime (schema) #1672
• How to de-register a runtime? #2089