Add a debugging techniques page to the C++ and Rust documentation

This is listed under the reference ("how to") and explains `SIXTYFPS_SLOW_ANIMATIONS`, SIXTYFPS_DEBUG_PERFORMANCE` as well as `SIXTYFPS_SCALE_FACTOR`.

cc #728

api/sixtyfps-rs/docs.rs

@@ -165,3 +165,8 @@ pub mod generated_code {
         }
     }
 }
+
+pub mod debugging_techniques {
+    #![doc = include_str!("docs/debugging_techniques.md")]
+    #![doc = ""]
+}