Allows to specify the port on which serve documentation (GH-31145)

[user@localhost]$ make serve      # default configuration, no change
python3 ../Tools/scripts/serve.py build/html
Serving build/html on port 8000, control-C to stop
^CShutting down.

[user@localhost]$ make serve SERVE_PORT=8080 # new option
python3 ../Tools/scripts/serve.py build/html 8080
Serving build/html on port 8080, control-C to stop