doc: mainpage.md: fix for new doxygen

I'm getting the following error on doxygen 1.9.1:

.../zephyr/doc/custom-doxygen/mainpage.md:1:
error: multiple use of section label 'index' for main page,
(first occurrence:
.../zephyr/doc/custom-doxygen/mainpage.md, line 1)
(warning treated as error, aborting now)

Extension error (zephyr.doxyrunner):
Handler <function doxygen_build at 0x7f1abf9e30d0>
for event 'builder-inited' threw an exception
(exception: Doxygen process returned non-zero (1))

This fixes it for me without breaking anything in
_build/doxygen/html/index.html that I can see.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>