* Use dark-light on Mac and Windows
dark-light has a nasty problem on Linux: https://github.com/frewsxcv/rust-dark-light/issues/17
So we made dark-light opt-in in https://github.com/emilk/egui/pull/1437
This PR makes dark-light a default dependency again,
but only use it on Max and Windows.
This is controlled with the new NativeOptions::follow_system_theme.
If this isn't enabled, then NativeOptions::default_theme is used.
* Add eframe::WebOptions
* Use old 1.51 toolchain instead of bleeding edge
1.52 and 1.53 has problems with incremental compilation,
so some people chose to stay on 1.51 for now.
So let's make sure egui supports 1.51 for a while!
* Update to cint 0.2.2 to get rust 1.51.0 compatability
* Deny doc errors
* Add intentional bad intradoc link to test CI
* Add cargo doc to CI
* Fix carg doc web (add wasm32 target)
* Fix intentionally broken doc-link