Add .NET C# quick start tutorial (github#412)

* Add C# dotnet quickstart

* Update dotnet.md

Fixing steps to have numbers, a few heading formatting issues, and changing the "step" text above each method.

* Apply suggestions from code review

Applying most changes. I want to verify that auto numbering will work across code sections before accepting those.

Co-authored-by: Lana Brindley <github@lanabrindley.com>

* Apply suggestions from code review

Co-authored-by: Lana Brindley <github@lanabrindley.com>

* Update dotnet.md

Co-authored-by: Ryan Booz <ryan@timescale.com>
Co-authored-by: Lana Brindley <github@lanabrindley.com>