Rollup merge of #80527 - jyn514:rustdoc-lints, r=GuillaumeGomez

Make rustdoc lints a tool lint instead of built-in

- Rename `broken_intra_doc_links` to `rustdoc::broken_intra_doc_links` (and similar for other rustdoc lints; I don't expect any others to be used frequently, though).
- Ensure that the old lint names still work and give deprecation errors
- Register lints even when running doctests
- Move lint machinery into a separate file
- Add `declare_rustdoc_lint!` macro

Unblocks https://github.com/rust-lang/rust/pull/80300, https://github.com/rust-lang/rust/pull/79816, https://github.com/rust-lang/rust/pull/80965. Makes the strangeness in https://github.com/rust-lang/rust/pull/77364 more apparent to the end user (note that `missing_docs` is *not* moved to rustdoc in this PR). Closes https://github.com/rust-lang/rust/issues/78786.

## Current status

This is blocked on #82620 (see https://github.com/rust-lang/rust/pull/80527#issuecomment-787401519)

compiler/rustc_span/src/symbol.rs

@@ -1021,6 +1021,7 @@ symbols! {
         rustc_then_this_would_need,
         rustc_unsafe_specialization_marker,
         rustc_variance,
+        rustdoc,
         rustfmt,
         rvalue_static_promotion,
         sanitize,