chore: make the document of the example able to be previewed

[kingwingfly]

Objective

We can only preview the output with things like cargo expand. In this PR, I add some lines about previewing documentation with cargo doc --open.

Solution

• Turn on a necessary feature gate in all_tuples example.
• Add annotation in debugging.md

Testing

The result of following cmd is almost the same as docs.rs

RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --example all_tuples --no-deps --open

[BenjaminBrienen]

GitHub needs to add a button to retrigger CI