Remove some duplication in the documentation of API that's shared between Rust and C++

This change makes the start of sharing the docs for the `TimerMode` enum
between Rust and C++. The reference to Timer::start in there works as
both doxygen and rustdoc find the right reference, but this needs
careful editing in the future and double-checking!

Another "caveat" is that the docs for the TimerMode enum say that the
enum is defined in the file "sixtyfps_generated_public.h", which is
correct as-is but not as pretty as "sixtyfps.h". I tried various ways
with \file and \includedoc, but couldn't get it working differently.

To implement this, the cppdocs steps now also runs cbindgen and cbindgen
generates a new sixtyfps_generated_public.h file that contains types we
do want to have in the public sixtyfps namespace.

api/sixtyfps-cpp/build.rs

@@ -20,5 +20,9 @@ fn main() -> Result<(), anyhow::Error> {
 
     println!("cargo:GENERATED_INCLUDE_DIR={}", output_dir.display());
 
-    cbindgen::gen_all(&root_dir, &output_dir)
+    let dependencies = cbindgen::gen_all(&root_dir, &output_dir)?;
+    for path in dependencies {
+        println!("cargo:rerun-if-changed={}", path.display());
+    }
+    Ok(())
 }