docs: Update doxygen config file

This patch updates doxygen config file to include
docs/platform_guide.md and docs/library_usage.md in
generated PDF document.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
diff --git a/docs/doxygen.cfg b/docs/doxygen.cfg
index 484090b..feb461b 100644
--- a/docs/doxygen.cfg
+++ b/docs/doxygen.cfg
@@ -792,6 +792,8 @@
 
 INPUT                  = @@SRC_DIR@@/README.md \
                          @@SRC_DIR@@/docs/contributing.md \
+                         @@SRC_DIR@@/docs/platform_guide.md \
+                         @@SRC_DIR@@/docs/library_usage.md \
                          @@SRC_DIR@@/docs/firmware \
                          @@SRC_DIR@@/docs/platform \
                          @@SRC_DIR@@/include \