Remove @page CSS

- This actually would cause print pages to to sometimes be shifted left
and the right site to be cut off a little or sometimes not be shifted.
It just caused un-reliable results for me.  Removing the gave a more
reliable experience.
Remove markdown2
Enable almost all extensions by default

- Don’t enable b64 since a global b64 is preferred, but this is left in
as an option if searching for images via regex ever proves to not be
sufficient
- Admonitions is not enabled since this is an experimental extension
and is not really handled in the current stylings
- Codehilite is not enabled by default since it is generally controlled
using settings switches. But can be manually added to override style or
language guessing or adding lines
Add info about highlighting's guess, line number, and style feature
codehilite support for noclasses option
Remove debug statements