Merge pull request #25161 from mshabunin:doc-upgrade-5.x

Documentation transition to fresh Doxygen (5.x) #25161 

Port of #25042

Merge with opencv/opencv_contrib#3687
CI part: opencv/ci-gha-workflow#162

modules/gapi/doc/00-root.markdown

@@ -41,6 +41,10 @@ G-API documentation is organized into the following chapters:
 
 - API Reference: functions and classes
 
+    - @subpage gapi_ref
+
+      Core G-API classes, data types, backends, etc.
+
     - @subpage gapi_core
 
       Core G-API operations - arithmetic, boolean, and other matrix
@@ -51,6 +55,14 @@ G-API documentation is organized into the following chapters:
       Image processing functions: color space conversions, various
       filters, etc.
 
+    - @subpage gapi_video
+
+      Video processing functionality.
+
+    - @subpage gapi_draw
+
+      Drawing and composition functionality
+
 # API Example {#gapi_example}
 
 A very basic example of G-API pipeline is shown below:

modules/gapi/doc/30-implementation.markdown

@@ -4,7 +4,7 @@
 
 # G-API Implementation details
 
-Note -- this section is still in progress.
+@note this section is still in progress.
 
 # API layer {#gapi_detail_api}