[Documentation] Create a sample/doc for long-running OBO scenario

[pmaytak]

Please check those that apply

• documentation doesn't exist
• needs example

Description of the issue
Create a sample/doc for long-running OBO scenario. It should show how to initiate (InitiateLonRunningProcessInWebApi) and acquire tokens (AcquireTokenInLongRunningWebApi) in a web API that performs a long-running process. Should be emphasized how to use a proper key like session ID (sid). Good real-world example could be using a Graph delta API, where web API subscribes to receive some delta information periodically from Graph about a user.

MSAL.NET wiki
Identity Web wiki
Use delta query to track changes in Microsoft Graph data
Get incremental changes to messages in a folder