Added deprecation notice for web intents.

Added deprecation notice for web intents.

Also added public template for app_intents.html doc back in so that previous bookmarks do not result in a 404.

Note that I've repeated the deprecation warning under each <h2> in case a developer winds up in a section via search. Eventually we will want to add an 'Instead, use...' sentence/paragraph once this is defined and at the same time
remove the bulk of content in place.

BUG=174262
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=182058

[mkearney1, 2013-02-12 18:32:47]

Hopefully this does the trick for now.

[benwells, 2013-02-12 21:26:45]

lgtm