chore: simplify Netlify config (#1541)

Previously, the Netlify configuration copied the docs into a
`_netlify_out` dir as a workaround for netlify/build-image#505. This
upstream issue was fixed back in January, so this workaround hasn't been
necessary for a while. Therefore, we can remove this extra complexity
from our configuration.

I also changed the build script to a multi-line string which I think is
a bit nicer.

netlify.toml

@@ -1,7 +1,10 @@
 [build]
-  command = "rustup install nightly --profile minimal && cargo doc --no-deps && cp -r target/doc _netlify_out"
-  environment = { RUSTDOCFLAGS= "--cfg docsrs -D warnings" }
-  publish = "_netlify_out"
+  command = """
+    rustup install nightly --profile minimal \
+      && cargo doc --no-deps
+    """
+  environment = { RUSTDOCFLAGS= "--cfg docsrs" }
+  publish = "target/doc"
 
 [[redirects]]
   from = "/"