Documentation - write tour of sp_Blitz

[BrentOzar]

Way back when, I wrote the stored proc with a written tour built into the code so you could understand what it was doing. Now, the intro part has gotten sufficiently complex, and people won't read through the whole time. Do a condensed version in Markdown instead, explaining how it works, as part of the onboarding for contributors.

[BrentOzar]

Added TOURSTOP comments, and blog post is in the queue. Holding blog post until Jan release of the First Responder Kit since folks will need to see the TOURSTOP comments.