Clarify how to use regular expressions

https://stackoverflow.com/questions/54081489/cucumber-3-how-to-define-a-regular-expression

Leads me to believe the docs do not cover this well.

1. Create page like cucumber-epressions for regex.
2. Mention in step def and cucumber expression pages.
3. Remove wikipedia links.

Start by reading the linked Stack Overflow question and the existing cucumber expressions, step definition, and cucumber expression pages. Add a dedicated regular-expression page, cross-reference it from the relevant pages, and remove the Wikipedia links; done means the requested regex guidance is discoverable and the links are gone.

Written by the indexing model from the issue text.