Add more documentation for keyboard types

[notgull]

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

This PR adds documentation for some of the items introduced in #2662

[kchibisov]

Could you resolve conflicts?

[maroider]

Since there's no neat workaround for rust-lang/rust#32104 still being unresolved, I guess linking to it through GitHub works fine for now. My gut tells me we should be linking to a tagged release rather than the master branch, but linking to master initially is fine. I did find a way to link to the local copy of the SVG first, and then fall back to the GitHub link (roughly <img alt="Standard 1 key" src="../../../../images/keyboard_standard_1_key.svg" onerror="this.src = 'https://raw.githubusercontent.com/rust-windowing/winit/master/images/keyboard_standard_1_key.svg'">), but that wouldn't link the local version of the image if Winit's docs are built as a dependency, though I suppose instances where that would matter are exceedingly rare, if nonexistent.

[maroider]

While I'm sorely tempted to have one of us look into the SVG use element as a means of further reducing the SVG size, I think the size is OK now. This looks good otherwise (merge conflicts notwithstanding).