Missing documentation for `hat_map.dtb` in the device tree docs

[flo-at]

The hat_map.dtb map file introduced in this commit is not currently mentioned anywhere in the device tree documentation.

It seems like it should be documented alongside overlay_map.dtb, likely in the same section of the docs (e.g. here).

As far as I understand it, hat_map.dtb maps HAT UUIDs (read from the EEPROM) to the corresponding device tree overlays during boot. However, I’m not familiar with the bootloader internals, so I’m not confident enough to document this myself.

Could this file be added to the documentation, along with an explanation of how it’s used?