.azurepipelines: Add Platform CI template

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2570



Add template for Platform CI steps for a Pytools based build.
Add README to describe the template and how to use it.
Add helpful information for working with azurepipelines, templates, and
lessons learned.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>

Reviewed-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Bret Barkelew <bret.barkelew@microsoft.com>