Improve Doxygen generated documentation for decode

Private functions are not included in Doxygen output
Deprecated functions are marked in Doxygen output
All the XxxxInMapX() functions are documented (briefly).
Fix some broken Doxygen references.

* Improve doxygen-generated documentation

* Finish up private and deprecated and doxygen errors for decode

* tiny comment clean up

---------

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/doc/Numbers.md b/doc/Numbers.md
index d47ffb8..cefd478 100644
--- a/doc/Numbers.md
+++ b/doc/Numbers.md
@@ -1,6 +1,4 @@
 
-@file Numbers.md
-
 @anchor BigNumbers
 
 # Big Numbers
diff --git a/doc/Serialization.md b/doc/Serialization.md
index 8637e47..6ee3d24 100644
--- a/doc/Serialization.md
+++ b/doc/Serialization.md
@@ -1,4 +1,3 @@
-@file Serialization.md
 
 @anchor Serialization
 
diff --git a/doc/Tagging.md b/doc/Tagging.md
index 7cbc95b..abb75f9 100644
--- a/doc/Tagging.md
+++ b/doc/Tagging.md
@@ -1,4 +1,3 @@
-@file Tagging.md
 
 @anchor CBORTags
 
diff --git a/doc/mainpage.dox b/doc/mainpage.dox
new file mode 100644
index 0000000..18c27f8
--- /dev/null
+++ b/doc/mainpage.dox
@@ -0,0 +1,15 @@
+/*! @mainpage QCBOR Documentation
+
+@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"
+
+Subject Matter
+- @ref SpiffyDecode
+
+*/