chore: deny warnings in docs builds

This way, we'll get a build failure on PRs if the docs are broken.
I really thought we already did this, but apparently, we don't?

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

netlify.toml

@@ -3,9 +3,14 @@
     rustup install nightly --profile minimal \
       && cargo doc --no-deps --all-features
     """
-  environment = { RUSTDOCFLAGS= "--cfg docsrs" }
   publish = "target/doc"
 
+[build.environment]
+  RUSTDOCFLAGS="""
+    -D warnings \
+    --cfg docsrs
+    """
+
 [[redirects]]
   from = "/"
   to = "/tracing"