ASoC: doc: dapm: extend initial descrption

Extend the initial description providing clearer definitions of "widget"
and "route", and to stop using the word "component" to mean "widget". Give
more details and clarify wording and add a picture representing a real DAPM
graph.

Group all the introductory paragraphs before the "DAPM power domains", and
split the latter to a specific section.

Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Link: https://lore.kernel.org/r/20240426-dapm-docs-v2-7-87b07547eb5b@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: lucaceresoli