docs: Fold the syntax guide into the reference manual

[RyanGlScott]

Currently, the documentation for how to use Cryptol is spread across the reference manual and the syntax guide. We don't really want to have both, since the reference manual is intended to contain all of the same information as the syntax guide (indeed, the reference guide was originally based on the syntax guide as a starting point, but has grown over time).

We should remove the syntax guide, making sure that any relevant information has a corresponding home in the reference manual.