Currently, rustdoc only displays `repr(transparent)` if the type’s
fields are public, so there is no indication in the documentation that
`NotNan` has `repr(transparent)`. Even if it was visible, there is not
currently a strong consensus that such a repr may be assumed to be a
stable part of the public API.

Therefore, add paragraphs to `NotNan` and `OrderedFloat`’s documentation
guaranteeing the repr. I also mentioned `bytemuck` so as to not be
encouraging unsafe code, though the phrasing isn’t great.

This gives a more strongly-typed way for users to take advantage of
`OrderedFloat`’s representation.

Document that `NotNan` is `repr(transparent)`.

Also remove a stray blank line that showed up in the documentation.

This should fix all deprecation warnings from GitHub Actions.

actions-rs/toolchain and actions-rs/cargo are both unmaintained, but
they can largely be replaced with plain `rustup` and `cargo` commands,
except for the problem matchers.

Document signed zero hazards.

CI: update actions/checkout, remove actions-rs/*

Signed-off-by: tison <[email protected]>

feat: integrate with derive-visitor