Add HDElkDiagramGen API documentation

source/SpinalHDL/Libraries/EDA/tools/hdelkdiagramgen.rst

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+
+HDElkDiagramGen
+===============
+
+``HDElkDiagramGen`` is a tool from the `SpinalSchemaGen <https://github.com/SpinalHDL/SpinalSchemaGen>`_ library that generates an interactive block diagram of a SpinalHDL design as an HTML file.
+After running elaboration, pass the ``SpinalReport`` returned by ``SpinalVerilog`` or ``SpinalVHDL`` to generate the diagram.
+Open the resulting HTML file in any modern web browser to explore the hierarchy.
+
+.. code-block:: scala
+
+   import spinal.schema.elk.HDElkDiagramGen
+
+   HDElkDiagramGen(SpinalVerilog(new MyToplevel))
+
+This will create a file named ``MyToplevel.html`` in the current working directory.
+
+Setup
+-----
+
+``HDElkDiagramGen`` lives in the separate ``spinalhdl-schema-gen`` library.
+Add the following dependency to your ``build.sbt``:
+
+.. code-block:: scala
+
+   libraryDependencies += "com.github.spinalhdl" %% "spinalhdl-schema-gen" % "0.0.4"
+
+Usage
+-----
+
+The tool accepts the ``SpinalReport`` produced by ``SpinalVerilog`` or ``SpinalVHDL``:
+
+.. code-block:: scala
+
+   import spinal.schema.elk.HDElkDiagramGen
+
+   // With Verilog output
+   HDElkDiagramGen(SpinalVerilog(new MyToplevel))
+
+   // With VHDL output
+   HDElkDiagramGen(SpinalVHDL(new MyToplevel))
+
+Output
+------
+
+The generated ``<TopLevelName>.html`` file contains an interactive block diagram for every module in the design hierarchy, rendered using the `HDElk <https://github.com/Readon/hdelk>`_ library.
+Open it in any modern web browser to navigate between diagrams.
+
+The diagram shows:
+
+* Input and output ports of each module
+* Internal signals and registers
+* Connections between sub-modules
+* Clock domain colouring (each clock domain is assigned a distinct highlight colour; mixed-clock modules are marked separately)