Add API description for HDElkDiagramGen

Do not merge until SpinalHDL/SpinalHDL#1079 has not been merged.
Due to SpinalHDL/SpinalHDL#1079 submission, a new application API method has been generated.
```scala
import spinal.lib.tools.HDElkDiagramGen
HDElkDiagramGen(SpinalVerilog(new _))
```
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