docs: Use doxygen config to mark the main page

The doxygen config option "USE_MDFILE_AS_MAINPAGE" can be used to set
the main page in doxygen generated pdf. This allows us to remove the
"#mainpage" from the README file which markdown doesn't parse.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>

README.md

@@ -1,4 +1,4 @@
-RISC-V Open Source Supervisor Binary Interface (OpenSBI) {#mainpage}
+RISC-V Open Source Supervisor Binary Interface (OpenSBI)
 ========================================================
 
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