Move syntax docs to the docs folder

[cknitt]

This is in preparation of

• moving the syntax sources to integrate them directly into the dune project
• collecting the developer documentation that is currently scattered across the repo into the docs directory

Also, no need to explicitly mention Dune and Reanalyze as prerequisites as they are installed via the "opam install" command a few lines below.

[cristianoc]

The docs dir does not really contain docs, but this is a start.