Migrate or link to gemspec documentation

[duckinator]

gemspecs are very well documented at https://guides.rubygems.org/specification-reference/ — but it was not obvious to me to check guides.rubygems.org instead of bundler.io.

I think it would make sense to either:

1. move the gemspec documentation to bundler.io, or
2. add a page that is basically "documentation for gemspecs is in rubygems.org" with a link to the guides.rubygems.org page