doc: release notes: description of SOC, DTS, BOARD, and ARCH roots

This commit updates the release notes with the changes introduced
in #26715.

It also informs readers that existing use of `$(SOC_DIR)` in Kconfig
must be updated, for example to use `rsource` instead of
`source $(SOC_DIR)` in order to adopt to latest changes.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
1 file changed