Docs: don't treat warnings as fatal

The latest version of odoc insists on generating the following warning

File ../../../libISA/.libISA.objs/byte/libISA.odoc:
Warning: While resolving the expansion of include at File libISA/isa_parser.mli, line 134, character 2
Hidden constructors in type 'LibISA.Isa_parser.MenhirInterpreter.element'

This warning relates to code generated by the menhir parser generator and there
seems to be no way to prevent the warning from being generated.

So I am reluctantly disabling the 'warnings are fatal' setting so that we can
avoid what I consider to be a odoc/menhir bug.

Author: alastairreid

libISA/dune

@@ -90,4 +90,4 @@
  (libraries menhirLib ocolor str yojson zarith z3))
 
 
-(env (_ (odoc (warnings fatal))))
+; (env (_ (odoc (warnings fatal))))