Write a user guide on fairness features.

docs/user-guide/SUMMARY.md

@@ -5,3 +5,4 @@
 - [Guide to the Optimizer API](./optimizer.md)
 - [Local Differential Privacy capabilities](./local_dp.md)
 - [Secure Aggregation capabilities](./secagg.md)
+- [Fairness capabilities](./fairness.md)

docs/user-guide/index.md

@@ -18,3 +18,5 @@ This guide is structured this way:
     Description of the local-DP features of declearn.
 - [Secure Aggregation capabilities](./secagg.md):<br/>
     Description of the SecAgg features of declearn.
+- [Fairness capabilities](./fairness.md):<br/>
+    Description of the fairness-aware federated learning features of declearn.