Add regex101.com reference to both RE functions

Ref rdwatters/hugo-docs-concept#97

content/functions/findRe.md

@@ -35,6 +35,8 @@ You can limit the number of matches in the list with a third parameter. The foll
 
 {{% note %}}
 Hugo uses Golang's [Regular Expression package](https://golang.org/pkg/regexp/), which is the same general syntax used by Perl, Python, and other languages but with a few minor differences for those coming from a background in PCRE. For a full syntax listing, see the [GitHub wiki for re2](https://github.com/google/re2/wiki/Syntax).
+
+If you are just learning RegEx, or at least Golang's flavor, you can practice pattern matching in the browser at <https://regex101.com/>.
 {{% /note %}}
 
 <!-- Removed per request of @bep: https://github.com/spf13/hugo/issues/3188 -->

content/functions/replacere.md

@@ -26,4 +26,6 @@ Replaces all occurrences of a regular expression with the replacement pattern.
 
 {{% note %}}
 Hugo uses Golang's [Regular Expression package](https://golang.org/pkg/regexp/), which is the same general syntax used by Perl, Python, and other languages but with a few minor differences for those coming from a background in PCRE. For a full syntax listing, see the [GitHub wiki for re2](https://github.com/google/re2/wiki/Syntax).
+
+If you are just learning RegEx, or at least Golang's flavor, you can practice pattern matching in the browser at <https://regex101.com/>.
 {{% /note %}}