Documentation/dac: add STM32H7 on-chip DAC driver docs

Covers basic write mode, DMA circular and stream modes with code examples,
data structures, ioctl commands, and Kconfig configuration.

Signed-off-by: Andrey Sobol <andrey.sobol.nn@gmail.com>

Author: seniorandre

Documentation/components/drivers/character/analog/dac/index.rst

@@ -83,3 +83,11 @@ SPI-based DACs:
 
   dac7554/index.rst
   mcp48xx/index.rst
+
+On-Chip DAC Drivers
+===================
+
+.. toctree::
+  :maxdepth: 1
+
+  stm32h7/index.rst