docs: dt: Make references and mention kernel abstractions

This document is named "Linux and Devicetree" so I think we
should write a bit more about how the device tree fits into
the Linux kernel frameworks these days after we have created
the fwnode and swnode.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
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ChangeLog v2->v3:
- Rebase on v6.13-rc1
- Resend because this seems to keep being ignored.
ChangeLog v1->v2:
- Spelling mistakes
- Resend because nothing happens