docs: fix a bunch of RustDoc warnings/errors (tokio-rs#1524)

This branch fixes some minor RustDoc issues. In particular:

- The `broken_intra_doc_links` lint was renamed to
  `rustdoc::broken_intra_doc_links`. This generates a warning, since the
  old name was deprecated.
- `ignore` code blocks are specifically for _Rust_ code that should not
  be compiled, not for other text blocks. We were using `ignore` on JSON
  blocks, which generates a warning.
- A bunch of links in `tracing-subscriber`'s RustDocs were broken. This
  fixes that.

I also changed the Netlify configuration to run with `-D warnings`, so that
we can surface RustDoc warnings in CI builds.

Signed-off-by: Eliza Weisman <[email protected]>

tracing-opentelemetry/src/lib.rs

@@ -97,7 +97,7 @@
     html_logo_url = "https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png",
     issue_tracker_base_url = "https://github.com/tokio-rs/tracing/issues/"
 )]
-#![cfg_attr(docsrs, deny(broken_intra_doc_links))]
+#![cfg_attr(docsrs, deny(rustdoc::broken_intra_doc_links))]
 
 /// Implementation of the trace::Layer as a source of OpenTelemetry data.
 mod layer;