Document the new LAYER.IDENTIFY block

[rouault]

added per MapServer/MapServer#7318

@geographika Do you have a source file for en/images/layer_classes.png ?

[geographika]

@rouault - I documented this at https://github.com/MapServer/MapServer-documentation/blob/main/scripts/mapscript-docs/ReadMe.rst#updating-the-mapscript-relationship-diagrams and used the script at https://github.com/MapServer/MapServer-documentation/blob/main/scripts/mapscript-docs/diagrams.py

I was testing on Windows, so if you have any problems let me know, and I can run and add the image to this PR.

[rouault]

I documented this

thanks but this diagrams.py script regenerates files in en/mapscript/mapscript-api/images/ and not en/images/layer_classes.png (furthermore when testing diagrams.py I get the error b"Warning: syntax ambiguity - badly delimited number '0..' in line 7 of /tmp/tmpph_aok7z/tmpm5cdewz1 splits into two tokens\nError: /tmp/tmpph_aok7z/tmpm5cdewz1: syntax error in line 7 near ']'\n from dot, which I pinpointed to be due to "0..1" labels that it doesn't like.)
Anyway that isn't critical

[geographika]

@rouault - sorry I pointed you to the MapScript diagram scripts. The MapServer class diagrams were created earlier as part of #196, with the script at https://github.com/geographika/mappyfile/blob/master/docs/scripts/class_diagrams.py
I have updated and added the image to this PR.

[rouault]

I have updated and added the image to this PR.

thanks!