doc: doxygen: use full path for USE_MDFFILE_AS_MAINPAGE

Otherwise some recent Doxygen versions (1.11.0) generate a warning.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

doc/zephyr.doxyfile.in

@@ -1105,7 +1105,7 @@ 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 = mainpage.md
+USE_MDFILE_AS_MAINPAGE = @ZEPHYR_BASE@/doc/_doxygen/mainpage.md
 
 # The Fortran standard specifies that for fixed formatted Fortran code all
 # characters from position 72 are to be considered as comment. A common