doc: add DFU AT documentation

This commit adds DFU AT commands documentation.
It also documents an example Python host script for DFU.

Signed-off-by: Syver Haraldsen <syver.haraldsen@nordicsemi.no>

Author: SyverHaraldsen

doc/app/at_commands.rst

@@ -36,6 +36,7 @@ The modem specific AT commands are documented in the `nRF91x1 AT Commands Refere
 
    at_generic
    at_cmux
+   at_dfu
    at_fota
    at_gnss
    at_icmp