Add Implementation Status to metadata, remove Polyfill (#769)

Consolidate resources and projects supporting the specification
under the custom "Other" header.

index.bs

@@ -13,8 +13,7 @@ Abstract: This document describes a dedicated low-level API for neural network i
 Repository: https://github.com/webmachinelearning/webnn
 Test Suite: https://github.com/web-platform-tests/wpt/tree/master/webnn
 Implementation Report: https://wpt.fyi/results/webnn?label=master&label=experimental&aligned&q=webnn
-!Explainer: <a href="https://github.com/webmachinelearning/webnn/blob/master/explainer.md">explainer.md</a>
-!Polyfill: <a href="https://github.com/webmachinelearning/webnn-polyfill">webnn-polyfill</a> / <a href="https://github.com/webmachinelearning/webnn-samples">webnn-samples</a>
+!Other: <a href="https://webmachinelearning.github.io/webnn-status/">Implementation Status</a>, <a href="https://github.com/webmachinelearning/webnn/blob/master/explainer.md">Explainer</a>, <a href="https://github.com/webmachinelearning/webnn-samples">Samples</a>
 Markup Shorthands: markdown yes
 Markup Shorthands: dfn yes
 Markup Shorthands: idl yes