doc: Fix Doxyfile USE_MDFILE_AS_MAINPAGE entry

The entry requires the relative path from where `doxygen` is invoked in
recent Doxygen versions. The change is backwards compatible with older
versions.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

Author: carlescufi

doc/Doxyfile

@@ -1090,7 +1090,7 @@ FILTER_SOURCE_PATTERNS =
 # (index.html). This can be useful if you have a project on for instance GitHub
 # and want to reuse the introduction page also for the doxygen output.
 
-USE_MDFILE_AS_MAINPAGE = main.md
+USE_MDFILE_AS_MAINPAGE = _doxygen/main.md
 
 # The Fortran standard specifies that for fixed formatted Fortran code all
 # characters from position 72 are to be considered as comment. A common