docs: deprecate cwa-risk-assessment

- add deprecation notice to document
- add reference to solution architecture

related to #453

cwa-risk-assessment.md

@@ -1,5 +1,21 @@
 # How does the Corona-Warn-App identify an increased risk?
 
+An up-to-date description of how the risk assessment and risk calculation in the Corona-Warn-App works is available in the [Solution Architecture > Mobile Application](/solution_architecture.md#mobile-applications).
+
+Please note that this document is _not_ up-to-date anymore. It reflects the original risk assessment for Corona-Warn-App Release < 1.8 (approximately December 2020) and is kept here for documentation purposes only.
+
+
+<br /><br /><br /><br />
+
+
+# DEPRECATION NOTICE
+
+**The below content is not up-to-date anymore!**
+
+<br /><br /><br /><br />
+
+
+
 ---
 **Later calculation improvements**