Strip comments from CRD descriptions

Kubernetes has some conventions for type descriptions:

- Line that only contains --- separates the description from comments.
  The leading lines are part of public documentation, the trailing lines are
  internal instructions such as code examples etc.
- Line that begins with TODO are internal notes for implementers, and not
  part of public documentation of the type.

This change strips above informational text away from the generated CRDs
while keeping them if they appear inside markdown fenced code block.

Signed-off-by: Tero Saarni <tero.saarni@est.tech>