Make a placeholder for developer's guide

Modified the mainpage in docs, and added a placeholder for future
developer's guide.

BUG=aomedia:2674

Change-Id: I0d86ed1dab3b225d4fdd77740d4f34eb46cff7d3
diff --git a/mainpage.dox b/mainpage.dox
index 6a6b11a..b4c61e8 100644
--- a/mainpage.dox
+++ b/mainpage.dox
@@ -1,8 +1,10 @@
-/*!\mainpage AOMedia Codec SDK
+/*!\mainpage AOMedia AV1 Codec
 
   \tableofcontents
 
-  \section main_intro Introduction
+  \section aom_sdk AOMedia Codec SDK
+
+  \subsection main_intro Introduction
   Welcome to the AOMedia Codec SDK. This SDK allows you to integrate your
   applications with the AOM and AV1 video codecs.
 
@@ -16,7 +18,7 @@
   \endif
 
 
-  \section main_startpoints Starting Points
+  \subsection main_startpoints Starting Points
   - Consult the \ref changelog for a complete list of improvements in this
     release.
   - \ref readme contains instructions on compiling the sample applications.
@@ -30,6 +32,18 @@
   \if decoder
   - \ref decoder reference
   \endif
+  <br>
+
+  \section av1_guide AV1 Developer's Guide
+
+  \if av1_encoder
+  - \ref encoder_guide
+  \endif
+
+  \if av1_decoder
+  - \ref decoder_guide
+  \endif
+  <br>
 
   \section main_support Support Options & FAQ
   The AOMedia project is an open source project supported by its community. For