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Add a documentation page for service manifests

This change adds a page describing the manifest file structure and
focuses on the options field that replaced "magic" service_manager
capabilities.

Bug: 866967
Change-Id: I4fbf53b8e3f821cb67c3133b21c8521a7536e429
Reviewed-on: https://chromium-review.googlesource.com/1227229Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593654}