Link to scitools-classroom organisation

[bjlittle]

📚 Documentation

A user recently suggested that it would be useful to reference the learning material in the sister organisation scitools-classroom.

@SciTools/iris-devs Are we happy to reference this material from the iris documentation?

Thumbs-up this issue if you agree, and discuss/suggest the most appropriate place to advertise this

[rcomer]

Maybe in the preamble to the user guide?
https://scitools-iris.readthedocs.io/en/stable/userguide/index.html#user-guide-index

[bjlittle]

Closed by #5609