Add guide for adding examples and enriching reference docs.

We should add a guide similar to [this](https://github.com/microsoftgraph/msgraph-sdk-powershell/wiki/Generation-Process) for adding examples and enriching/customizing reference docs. 
[AB#9466](https://microsoftgraph.visualstudio.com/0985d294-5762-4bc2-a565-161ef349ca3e/_workitems/edit/9466)