docs: fix how to contribute link

Issue-ref: resolves #6

Author: Lars Bauhofer

docs/get_involved.rst

@@ -15,6 +15,8 @@
 .. role:: underline
     :class: underline
 
+.. _get_involved:
+
 Get involved
 ============
 

docs/index.rst

@@ -211,4 +211,4 @@ The people working in this project are listed `here <https://projects.eclipse.or
 How to contribute
 =================
 
-For contribution How-To please check the contribution guide `contribution guide <https://github.com/eclipse-score/score/blob/main/CONTRIBUTION.md>`_.
+For contribution How-To please check the contribution guide :ref:`get_involved`.