Big documentation update, particularly tags (#290)

* Big documentation update, particularly tags

* More documentation fixes

---------

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/doc/mainpage.dox b/doc/mainpage.dox
index 18c27f8..2a723ce 100644
--- a/doc/mainpage.dox
+++ b/doc/mainpage.dox
@@ -3,13 +3,28 @@
 @par Table of Contents
 
 API Reference
-- Encoding functions: @ref inc/qcbor/qcbor_encode.h "qcbor_encode.h"
-- Main/Basic decode functions: @ref inc/qcbor/qcbor_main_decode.h "qcbor_main_decode.h"
-- Spiffy decode functions: @ref inc/qcbor/qcbor_spiffy_decode.h "qcbor_spiffy_decode.h"
-- Tag decode functions: @ref inc/qcbor/qcbor_tag_decode.h "qcbor_tag_decode.h"
-- Number decode functions: @ref inc/qcbor/qcbor_number_decode.h "qcbor_number_decode.h"
+- Common
+   - Error codes and common constants: @ref inc/qcbor/qcbor_common.h "qcbor_common.h"
+- Encoding
+   - Main/Basic encode functions: @ref inc/qcbor/qcbor_main_encode.h "qcbor_main_encode.h"
+   - Number encode functions: @ref inc/qcbor/qcbor_number_encode.h "qcbor_number_encode.h"
+   - Tag encode functions: @ref inc/qcbor/qcbor_tag_encode.h "qcbor_tag_encode.h"
+- Decoding
+   - Main/Basic decode functions: @ref inc/qcbor/qcbor_main_decode.h "qcbor_main_decode.h"
+   - Spiffy decode functions: @ref inc/qcbor/qcbor_spiffy_decode.h "qcbor_spiffy_decode.h"
+   - Tag decode functions: @ref inc/qcbor/qcbor_tag_decode.h "qcbor_tag_decode.h"
+   - Number decode functions: @ref inc/qcbor/qcbor_number_decode.h "qcbor_number_decode.h"
+
+Note: the API Reference is largely complete, the subject matter below is partial
 
 Subject Matter
-- @ref SpiffyDecode
+- @ref Overview "QCBOR overview and implementation limits"
+- @ref Building "Building QCBOR with make and cmake"
+- @ref CodeSize "Minimizing the amount of code linked and disabling features"
+- @ref SpiffyDecode "The 'spiffy' decode functions for easier map decoding"
+- CBOR Numbers
+   - @ref BigNumbers "Characteristics, encoding and decoding big numbers"
+- Tags
+   - @ref CBORTags
 
 */