doc: add table of contents

Add table of contents to already great documentation.

BUG=none
TEST=rendered in gerrit

Change-Id: I27a5c44c4c6b9bfa3f87d0a0a6032b52568b53f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2209439
Commit-Queue: Jett Rink <jettrink@chromium.org>
Commit-Queue: Paul Fagerburg <pfagerburg@chromium.org>
Tested-by: Jett Rink <jettrink@chromium.org>
Auto-Submit: Jett Rink <jettrink@chromium.org>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
1 file changed