Document different formatters and formatter API

[mlvandijk]

The Reporting page (https://docs.cucumber.io/cucumber/reporting/) could use some attention:

• List all built in reporters
• Provide (links to) formatter API on how to build a custom formatter? Preferably per programming language:

• [x] ruby (added in #249)
• [ ] java
• [ ] javascript
• [ ] dotnet
• [ ] kotlin (nice to have)

This looks useful: https://relishapp.com/cucumber/cucumber/docs/formatters

More information on Custom Formatters: https://app.cucumber.pro/projects/cucumber-ruby/documents/branch/master/features/docs/extending_cucumber/custom_formatter.feature

[danascheider]

See also: https://github.com/cucumber/cucumber-ruby/issues/1294