doc: fix API index

doc: fix API index

With Doxygen 1.8.18, a warning appears when tagging
the main markdown header with {#index}.
That's why the tag has been removed from the API index in DPDK 20.05.
Unfortunately it makes the index page classified as a standard
"related page" instead of being the "main page".

The tag {#mainpage} could be used instead of {#index}.
Another solution, chosen here, is to specify the main page file
in the Doxygen configuration with the variable USE_MDFILE_AS_MAINPAGE.

Fixes: 76fb8fc486f9 ("doc: fix build with doxygen 1.8.18")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Tested-by: David Marchand <david.marchand@redhat.com>