doc: Rename #index label to #mainpage

For some reason, newer versions of doxygen started to complain about
the following:

 warning: multiple use of section label 'index' for main page

To mitigate, use alternative name for the label.

Signed-off-by: Michal Berger <michalx.berger@intel.com>
Change-Id: I6387a7c644c6626dd646c9fdd0e9ed1d430881f2
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/7132


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Community-CI: Mellanox Build Bot