Show DLR documentation for a specific expression type / node type

[zspitz]

The DLR documentation can be found at https://github.com/IronLanguages/dlr/tree/master/Docs.

It would be nice to show this documentation from the node context menu.

• Parse the expression tree spec and the DLR overview documents into Markdown.
• Request DLR maintainers to add it to the repo wiki (Surface DLR docs for deep linking IronLanguages/dlr#246)

I think only expr-tree-spec and perhaps some parts of dlr-overview are relevant.