Add web_aspect_ratio example

[rukai]

• [x] Tested on all platforms changed
• [x] Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• [x] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [x] Created or updated an example program if it would help users understand this functionality
• [x] Updated feature matrix, if new features were added or implemented

Adds a new example that creates a web canvas that fills an aspect-ratio'd div The main point of this example is to demonstrate the functionality provided by https://github.com/rust-windowing/winit/pull/2074 The example can land before or after, but landing it before will make it easier to test #2074

Another benefit of this example is demonstrating how to create a web canvas that fills up the available space while maintaining the same aspect ratio. Although this is slightly out of scope of winit I believe it would be extremely useful to provide as figuring out what combination of html elements and css configuration I needed to get this to work was surprisingly difficult for me even though I've been dabbling with web technologies for ages.

Pinging @liamolucko for feedback as this relates to your PR.