build DOC prepare framework for optional building of internal documentation

Prepare framework to put documentation about internal libyang processes
(intended for libyang developers, not its users) into the generated
doxygen documentation. The internal documentation will be available in
the Devel build, Release build is with user documentation only.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 83dfafa..6c82849 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -36,9 +36,11 @@
 if((CMAKE_BUILD_TYPE STREQUAL debug) OR (CMAKE_BUILD_TYPE STREQUAL Package))
     option(ENABLE_BUILD_TESTS "Build tests" ON)
     option(ENABLE_VALGRIND_TESTS "Build tests with valgrind" ON)
+    set(INTERNAL_DOCS YES)
 else()
     option(ENABLE_BUILD_TESTS "Build tests" OFF)
     option(ENABLE_VALGRIND_TESTS "Build tests with valgrind" OFF)
+    set(INTERNAL_DOCS NO)
 endif()
 option(ENABLE_COVERAGE "Build code coverage report from tests" OFF)
 
diff --git a/Doxyfile.in b/Doxyfile.in
index 5bb0916..2288465 100644
--- a/Doxyfile.in
+++ b/Doxyfile.in
@@ -506,7 +506,7 @@
 # will be excluded. Set it to YES to include the internal documentation.
 # The default value is: NO.
 
-INTERNAL_DOCS          = NO
+INTERNAL_DOCS          = @INTERNAL_DOCS@
 
 # If the CASE_SENSE_NAMES tag is set to NO then doxygen will only generate file
 # names in lower-case letters. If set to YES, upper-case letters are also
diff --git a/src/libyang.h b/src/libyang.h
index 50507a3..26fa65d 100644
--- a/src/libyang.h
+++ b/src/libyang.h
@@ -83,10 +83,20 @@
  * - @subpage howtoxml
  * - @subpage howtothreads
  * - @subpage howtologger
+ * - @subpage howtoplugins
  * - @subpage howtostructures
  */
 
 /**
+ * @internal
+ * @page internals Developers' Notes
+ * @tableofcontents
+ *
+ * Following texts describes various internal subsystems and mechanism in libyang which are hidden from external users, but important
+ * for libyang developers. The texts should explain various decisions made and internal processes utilized in libyang.
+ */
+
+/**
  * @page howtostructures Data Structures
  *
  * @section sizedarrays Sized Arrays