[mlir][docs] Remove the BuiltinDialect documentation from langref and generate it from ODS

Now that all of the builtin dialect is generated from ODS, its documentation in LangRef can be split out and replaced with references to Dialects/Builtin.md. LangRef is quite crusty right now and should really have a full cleanup done in a followup.

Differential Revision: https://reviews.llvm.org/D98562