Add a guide on how to use the .NET Octokit SDK

[skedwards88]

The GitHub docs now include a guide about how to use GitHub's JavaScript Octokit SDK to use the REST API: https://docs.github.com/rest/guides/scripting-with-the-rest-api-and-javascript

We would like to add a similar guide for the .NET Octokit SDK.

The guide should:

• Follow the same format as https://docs.github.com/rest/guides/scripting-with-the-rest-api-and-javascript. You can copy this guide to use as a template, but please be sure to update all JavaScript-specific content.
• Follow best practices for .NET, including adhering to a standard .NET linter
• Not recommend OAuth Apps, but instead recommend GitHub Apps