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Add add_new_policy.md as an enterprise doc.

All contents are copied from
https://sites.google.com/a/chromium.org/dev/developers/how-tos/enterprise/adding-new-policies.
Note that even some contents in the original link are out of date, this
CL does not update any of them.

Bug: 1022143
Change-Id: I117da6dba6b56cfa41538756f6eb2a4d92d8325d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902274
Commit-Queue: Owen Min <zmin@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714937}