Use `#[doc(alias = "...")]`

[madsmtm]

We should use #[doc(alias = "...")] on Code/Key variants, to make things easier to search for.

Examples of where this would be helpful:

• Note alternative names of Code::Backquote and Code::Quote #61
• Enter (alias Return)
• Backspace (alias Delete)
• Meta (alias Cmd/Command, Win/Windows and maybe Super)

[pyfisch]

Backspace (alias Delete)

Since there is an actual key called "Delete" this would be confusing. We shouldn't make the same mistake as Apple here.