Add nestedtext-ruby link in related_projects.rst

[erikw]

Adding a link to nestedtext-ruby.

While there are still tons of code improvements and features that I would like to add, it's already usable and released to rubygems.org! All official tests are passing ✅

I will contribute with my Ruby helper for the official tests too, once I've cleaned up the code with linters and so on

https://github.com/erikw/nestedtext_tests/blob/add_ruby_api/api/nestedtext_official_tests.rb

[KenKundert]

I am in the process of making some changes, but should check in a new version in a few days. I expect to do a formal release in a few weeks. Your notice will show up in the official documentation at that point.

Thanks for creating this implementation.