On-prem docs

[zdrawku]

Adding docs for:

• Setting up Custom Authentication Providers - Related to Create documentation for setting up Custom Authentication Providers app-builder#41
• Exposing the environment config properties - Create a documentation on Configuration flags for On-Prem deployment app-builder#42
• Whitelisting external resources - Create a list of external references if required for whitelisting as part of a public document app-builder#43