Rollup merge of #60938 - jonas-schievink:doc-include-paths, r=petrochenkov

Rollup merge of #60938 - jonas-schievink:doc-include-paths, r=petrochenkov

rustdoc: make #[doc(include)] relative to the containing file

This matches the behavior of other in-source paths like `#[path]` and the `include_X!` macros.

Fixes https://github.com/rust-lang/rust/pull/58373#issuecomment-462349380
Also addresses https://github.com/rust-lang/rust/issues/44732#issuecomment-467660239

cc #44732

This is still missing a stdsimd change (https://github.com/jonas-schievink/stdsimd/commit/42ed30e0b5fb5e2d11765b5d1e1f36234af85984), so CI will currently fail. I'll land that change once I get initial feedback for this PR.