regs: Reorganize regulators documentation

Reorganize the existing phosphor-regulators application documentation.
Break the documentation into smaller files describing one topic.

This will make it e

regs: Reorganize regulators documentation

Reorganize the existing phosphor-regulators application documentation.
Break the documentation into smaller files describing one topic.

This will make it easier to add new documentation describing the
upcoming changes for multiple chassis systems.

Most of the 'changes' in this commit are just text that has been moved
between files.

Change-Id: I096f1d396910987188fe1143f36bade61655e7d1
Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>

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