Add public documentation for enterprise.hardwarePlatform extension API

Bug: 1008005
Change-Id: I8b4361012430447010ec98bf19cddf79638378fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824107Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700147}

chrome/common/extensions/docs/templates/public/extensions/enterprise_hardwarePlatform.html

+{{+partials.standard_extensions_api api:apis.extensions.enterprise_hardwarePlatform intro:intros.enterprise_hardwarePlatform/}}