docs: Document new interpolation function md5

website/source/docs/configuration/interpolation.html.md

@@ -161,6 +161,9 @@ The supported built-in functions are:
 
   * `lower(string)` - Returns a copy of the string with all Unicode letters mapped to their lower case.
 
+  * `md5(string)` - Returns a (conventional) hexadecimal representation of the
+    MD5 hash of the given string.
+
   * `replace(string, search, replace)` - Does a search and replace on the
       given string. All instances of `search` are replaced with the value
       of `replace`. If `search` is wrapped in forward slashes, it is treated