Use `rubydoc.info` for API documentation

[faraazahmad]

The current documentation for syntax_tree uses RDoc. However, it is not the most user-friendly docs tool out there, as you have also stated in #279. I propose syntax_tree use rubydoc.info for its API documentation going forward, and the syntax_tree website can link to it, white it transitions into a proper guide with tutorials and such.

https://rubydoc.info/gems/syntax_tree

[kddnewton]

The purpose was I didn't want to introduce a new dependency beyond what was in the standard library. I think if we want to improve the documentation, we should improve the RDoc template.