doc: add release not entry for mTLS authentication support

Author: vlaci

doc/manual/rl-next/mtls-substituter.md

@@ -0,0 +1,13 @@
+---
+synopsis: Support substituters using mTLS (client certificate) authentication 
+issues: []
+prs: [13030]
+---
+
+Added support for `ssl-cert` and `ssl-key` options in substituter URLs.
+
+Example:
+
+    https://substituter.invalid?ssl-cert=/path/to/cert.pem&ssl-key=/path/to/key.pem
+
+When these options are configured, Nix will use this certificate/private key pair to authenticate to the server.