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

@@ -7,6 +7,7 @@
 
 [build.environment]
   RUSTDOCFLAGS="""
+    -D warnings \
     --cfg docsrs \
     --html-before-content /opt/build/repo/assets/warning.html \
     --html-in-header /opt/build/repo/assets/noindex.html \