Add a high-level description of the API first on /syntax.html

[choldgraf]

I found it a little bit confusing that the first example starts with {proof:proof}. I think if there were a short section like "all directives in this package follow the pattern {proof:<title>} or something like this, I may have been a bit less-confused. Just sharing in case it's helpful!

[najuzilu]

How's this:

This package utilizes a Sphinx domain - named "proof" - to describe and link typeset markup objects (theorems, proofs, corollaries, etc.) which we think belong together. All directives follow the pattern {<domain_name>:<typeset>} while all the roles follow the pattern {<domain_name>:ref}. To utilize any directive in the proof domain follow the pattern {proof:<typeset>}. To reference any directive follow the pattern {proof:ref}.

[najuzilu]

Closing since this has been addressed here #39.