Move record syntax from Syntax.md to Record.md

[milesfrain]

There's some overlap [between Record.md and] the language syntax documentation:
https://github.com/purescript/documentation/blob/837ec4ff3a956cdeda31997bf5f697ee738295b6/language/Syntax.md#additional-forms-with-records
I think we should merge that content into this [Record.md] and then update the language syntax file to link [Record.md]. It makes more sense for that content to be in this section and just a link be in that file.

#330 (comment)