Modify the Gremlin Variants documentation to account for gremlin-language

With gremlin-language moving toward the canonical representation of Gremlin it probably should be documented especially since it will likely replace gremlin-groovy in Gremlin Server. While close to groovy, there are some differences worth documenting and some background information to likely change and include. Probably just need to add a new section called "Gremlin-Lang" and follow the pattern we've established in the other variants.