Merge pull request #25042 from mshabunin:doc-upgrade

Documentation transition to fresh Doxygen #25042

* current Doxygen version is 1.10, but we will use 1.9.8 for now due to issue with snippets (https://github.com/doxygen/doxygen/pull/10584)
* Doxyfile adapted to new version
* MathJax updated to 3.x
* `@relates` instructions removed temporarily due to issue in Doxygen (to avoid warnings)
* refactored matx.hpp - extracted matx.inl.hpp
* opencv_contrib - https://github.com/opencv/opencv_contrib/pull/3638

modules/gapi/doc/00-root.markdown

@@ -17,7 +17,7 @@ is volatile at the moment and there may be minor but
 compatibility-breaking changes in the future.
 
 # Contents
-
+gapi
 G-API documentation is organized into the following chapters:
 
 - @subpage gapi_purposes
@@ -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: