libyang CHANGE add LY_VALUE_LYB

Add a separate value format of the LYB representation of the type.
Some of the callbacks API of the type plugins was modified to reflect the
need of passing generic (binary) data representation of the value.
diff --git a/src/tree_data.h b/src/tree_data.h
index f32343f..4992b02 100644
--- a/src/tree_data.h
+++ b/src/tree_data.h
@@ -79,6 +79,7 @@
  * - @subpage howtoDataWD
  * - @subpage howtoDataManipulation
  * - @subpage howtoDataPrinters
+ * - @subpage howtoDataLYB
  *
  * \note API for this group of functions is described in the [Data Instances module](@ref datatree).
  *
@@ -357,6 +358,16 @@
  */
 
 /**
+ * @page howtoDataLYB LYB Binary Format
+ *
+ * LYB (LibYang Binary) is a proprietary libyang binary data and file format. Its primary purpose is efficient
+ * serialization (printing) and deserialization (parsing). With this goal in mind, every term node value is stored
+ * in its new binary format specification according to its type. Following is the format for all types with explicit
+ * support out-of-the-box (meaning that have a special type plugin). Any derived types inherit the format of its
+ * closest type with explicit support (up to a built-in type).
+ */
+
+/**
  * @ingroup trees
  * @defgroup datatree Data Tree
  * @{