PM: runtime: Expand introduction with core concepts and structure

I commonly see people have difficulty learning how runtime PM works
because of the following key points [*]:

 1) there are several boolean concepts in runtime PM, with somewhat
    similar meanings:

     enabled / disabled
     active / suspended
     allowed / forbidden

 2) if these concepts are documented at all, they're scattered across
    the kerneldoc or Documentation/

 3) the runtime_pm.rst docs don't make any attempt to ease a reader into
    understanding the concepts, and instead jump straight into how it's
    implemented (queues, 'struct device' fields, helpers).

Let's try to remedy this a bit by discussing the core concepts and
highlights at the top of the introduction, and introduce a few
sub-headings, so it's easier to navigate different aspects of the
introduction.

While shuffling the intro around, I also see that the existing text
largely mirrors the layout of the following sections (2, 3, and 4), but
does so out of order. Reorder those, and point to section numbers.

[*] In addition to API complexity. I count 61 pm_*() helpers, 7 of which
    are variations of put() and 8 of which are variations of get().

Signed-off-by: Brian Norris <briannorris@chromium.org>
Link: https://patch.msgid.link/20261001085009.v4.1.I45a794e4452e3df22419acdd8f11e2b788b3413f@changeid
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

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