docs: Complete the relative path to reference conf

It seems that generated page does not link correctly to document on
reference configuration without adding the complete path to `.rst` file
in the manual's `.md` file.

Author: Bouncheck

manual/core/README.md

@@ -71,7 +71,7 @@ variants that return a `CompletionStage`).
 customization is done through the driver configuration (refer to the
 [corresponding section](configuration/) of this manual for full details).
 
-We recommend that you take a look at the [reference configuration](configuration/reference/) for the
+We recommend that you take a look at the [reference configuration](configuration/reference/README.rst) for the
 list of available options, and cross-reference with the sub-sections in this manual for more
 explanations.