Move syntax docs to the docs folder (#5818)

Author: cknitt

res_syntax/cli/JSXV4.md renamed to docs/JSXV4.md

@@ -20,10 +20,10 @@ J. Blow.
 Required:
 
 - OCaml 4.10 or later
-- Dune
-- Reanalyze
 - OS: macOS, Linux or Windows
 
+The syntax source code is located in the directory `res_syntax`. All following commands are meant to be executed in that directory.
+
 ```sh
 opam install . --deps-only --with-test
 make # or "dune build"