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[Extension Docs] Content Script Update

Restructured and updated doc on content scripts.
Included more code samples and further explained
different script insertion types.

Change-Id: I93bf8d111078eb34e78ffd1a1fbae696c521c019
Reviewed-on: https://chromium-review.googlesource.com/698290
Commit-Queue: Crystal Lambert <crystallambert@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553866}