docs: create section to explain how to use private config (in instance folder) and .env

[rehmanis]

[akshitadixit]

Please provide a description.

[rehmanis]

@akshitadixit sorry this one requires to write about the current config setup. Since I wrote the current config structure, I will be working on writing about it as it requires some research and explanations on why I chose the current design approach. However, if you want I, will ping you here so you can review my PR once I make it.