Extend regex backtracking topic to link to non backtracking engine

[danmoseley]

We have a doc describing backtracking in regular expressions, potential pitfalls and how to control it:

https://github.com/dotnet/docs/blob/main/docs/standard/base-types/backtracking-in-regular-expressions.md

Since this was written there was a huge step forward -- in .NET 7 we shipped a non backtracking regex engine.
https://devblogs.microsoft.com/dotnet/regular-expression-improvements-in-dotnet-7/#backtracking-and-regexoptions-nonbacktracking

We should update this doc to point to that and when one might want to use that engine.

cc @stephentoub