Adding cosmos db .net6 lesson

[madebygps]

PR Checklist

Please use this checklist to track the completeness of your PR. We're always happy to assist you in completing these steps!

For more information see the Contributing guide.

• [ x] Prerequisites README.md is complete.
• [ x] Lesson README.md is complete.
• [ x] Source code is complete (one project with several exercises).
• [ x] Lesson Workspace is working (check Win+macOS).
• [x ] Link to the lesson is added to tables in Root & Lessons READMEs.
• A CodeTour is available for each exercise in the lesson.
• The CodeTours are working in the lesson Workspace (check Win+macOS).
• [ x] GitHub issue is linked. Fixes: #ISSUENR.

The only thing I didn't get to do was CodeTours, I could add later on, but wanted to get this in before summer gets hectic for me and I go on a long vacation lol