Fix devicetree documentation for new API

[mbolivar-nordic]

The devicetree documentation contains errors and is missing descriptions due to late-breaking semantic changes which had to go in before code freeze. This issue tracks the necessary work I'm aware of. Release notes are tracked separately in #22637.

• DTC_OVERLAY_FILE description says colon-separated, that's wrong (Devicetree docs updates for 2.3, part 1 #25470)
• centralize documentation of chosen zephyr nodes in a non-legacy file, provide a reference to them from the intro (Devicetree docs updates for 2.3, part 1 #25470)
• review doxygen docstrings and correct errors for generic APIs (Devicetree docs updates for 2.3, part 1 #25470)
• add introductory text to each section in the API reference (Devicetree docs updates for 2.3, part 1 #25470)
• review doxygen docstrings and correct errors for instance APIs (Dt docs for 2.3 part 2 #25633)
• review doxygen docstrings and correct errors for hw specific APIs (Dt docs for 2.3 part 2 #25633)
• address outstanding comments from DTS API close out for 2.3 #25009 (Dt docs for 2.3 part 2 #25633)
• review user guides and correct errors (Dt docs for 2.3 part 2 #25633)
• write a migration guide for moving to the new API
• check if zephyr v2.2 restriction in https://docs.zephyrproject.org/latest/guides/dts/bindings.html#bindings-file-syntax is still an issue