In gallery consider moving ORCA plotting example from general to oceanography

[jonseddon]

In #3710 it was suggested that @abooton is reviewing the documentation and galley examples. As part of this, is it worth considering moving the ORCA plotting example (https://scitools.org.uk/iris/docs/latest/examples/General/orca_projection.html) from the General section to the Oceanography section of https://scitools.org.uk/iris/docs/latest/gallery.html so that it's easier for people working with ocean data to find?

[tkknight]

The new Read The Docs style documentation has now been merged to master (not released), see #3752

You can see the gallery via: https://scitools-iris.readthedocs.io/en/latest/generated/gallery/index.html

I am happy to get this moved, I will create a PR.

[jonseddon]

Amazing, thanks to everyone for doing that!