Update instructor guide

[bast]

In particular we need documentation on how to set up:

• https://github.com/coderefinery/recipe-book
• https://github.com/coderefinery/recipe-book-recorded

From:

• https://github.com/coderefinery/recipe-book-template

Using:

• "git clone --mirror and git push"
• NOT using "generate from template" (as this would flatten history)

[bast]

We also need to document how to port important changes back to template.

[bast]

See git-intro guide on how this is set up.