Sphinx domain and autodoc for Antlr4 grammars

 A4Doc is a sphinx plugin for documenting Antlr4 grammars.
 .
 It’s primary target is to provide some overview for DSL users
 (generated documentation may not include some nuances essential
 for compiler developers).
 .
 A4Doc's features are:
 .
 * a new domain with grammar and rule directives called ``a4``;
 * directives for rendering railroad diagrams;
 * directive for extracting documentation comments and rendering docs and
   diagrams from `.g4` source files.