ci: Disallow warnings in rustdoc and test private items (#2341)

Make sure `cargo doc` runs cleanly without any warnings in the CI - some
recently introduced but still allowing a PR to get merged.

In case someone wishes to add docs on private items, make sure those
adhere to the same standards.

src/platform_impl/windows/event_loop.rs

@@ -333,7 +333,7 @@ impl<T> EventLoopWindowTarget<T> {
 /// entrypoint.
 ///
 /// Full details of CRT initialization can be found here:
-/// https://docs.microsoft.com/en-us/cpp/c-runtime-library/crt-initialization?view=msvc-160
+/// <https://docs.microsoft.com/en-us/cpp/c-runtime-library/crt-initialization?view=msvc-160>
 fn main_thread_id() -> u32 {
     static mut MAIN_THREAD_ID: u32 = 0;