doc: extensions: doxyrunner: fix HTML output handling
doc: extensions: doxyrunner: fix HTML output handling

The extension was not evaluating the GENERATE_HTML option correctly. The
get_doxygen_option returns a `List[str]`, not a `str`.

This effectively means that the Zephyr apidoc has not been updated for a
while as the extension was not moving the output to the final
destination folder.

Signed-off-by: Gerard Marull-Paretas <bedb45f43a286e49f4dcda039a436b216ae61ef3@nordicsemi.no>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr