ASoC: doc: dapm: describe how widgets and routes are registered

ASoC: doc: dapm: describe how widgets and routes are registered

The small paragraph describing how to register widgets is incomplete (does
not mention routes) and mentions snd_soc_dapm_new_control() which is not
really used. Moreover it appears as a part of the "Virtual Widgets"
subsection.

Replace it with a detailed and current description of how widgets and
routes are registered, mentioning both static declaration and runtime
registration. Also make this a section on its own.

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-v2-8-87b07547eb5b@bootlin.com
Signed-off-by: Mark Brown <broonie@kernel.org>