DOC: Fix diversity statement link in code of conduct

Fix diversity statement link in code of conduct:
- Add the file extension so that the link works across websites (the
  current solution not working on GitHub).
- Link to the specific diversiy statement section within the target
  file.

Documentation/docs/contributing/code_of_conduct.md

@@ -357,7 +357,7 @@ Harassment will not be tolerated in any of its forms either. Specifically:
 
 Although the above list cannot be exhaustive, we will not tolerate
 discrimination based on any of the protected characteristics in our
-[Diversity Statement](motivation).
+[Diversity Statement](motivation.md#diversity-statement).
 
 The relative anonymity (if any) in the discussion forums is no shelter for
 unacceptable behavior.