rustdoc: Built-in macros are not documented in all the necessary locations

[petrochenkov]

NOTE: Some cases below assume #63056 has landed.

• ❎ https://doc.rust-lang.org/nightly/core/default/index.html doesn't contain the derive macro Default.
  ✔️ Compare with the https://doc.rust-lang.org/nightly/std/fmt/index.html page which contains the derive macro Debug because it's introduced through a reexport rather than directly.

• ❎ Some built-in macros not available through the libcore root are documented in the root instead (e.g. derives) - https://doc.rust-lang.org/nightly/core/index.html.

• ✔️ libcore prelude is documented correctly https://doc.rust-lang.org/nightly/core/prelude/v1/index.html

• ❎ https://doc.rust-lang.org/nightly/std/default/trait.Default.html doesn't contain the derive macro Default.
  ❎ Note that https://doc.rust-lang.org/nightly/std/fmt/index.html doesn't contain the derive macro Debug as well.

• ❎ Derive and attribute macros in libstd prelude https://doc.rust-lang.org/nightly/std/prelude/v1/index.html has to be marked with #[doc(hidden)] currently because otherwise they generate dead links failing the linkchecker testing.
  As a result they do not appear on the page.
  What is worse, due to #[doc(hidden)] some traits with names matching derives (e.g. Default) are also not documented at that location.

[petrochenkov]

Looks like everything mentioned in this issue was fixed, and #83155 removes the #[doc(hidden)] attributes as well.

[pierwill]

@jyn514 Can we close this?

[jyn514]

My understanding is that the macros with doc(hidden) still aren't documented anywhere, since #83155 was closed.

[petrochenkov]

All the issues here are fixed now.