regex module: much much more documentation

[kris-watts-gravwell]

Enhancement

Regex is hard and our RE2 engine isn't as ubiquitous as others. More docs needed

What is the enhancement to be made?

More info about extractions, names, etc...

Also copy this spec in wholesale: https://github.com/google/re2/wiki/Syntax

Why should we make this change? (Business justification? What problem is the feature trying to solve?)

We should not have to point people at different links for specs, make regex docs one stop shopping

Any other comments?

that wiki link is BSD-3-Clause licensed so we can literally copy/paste the whole thing in with a license link at the top.