Documentation: Improve visibility of the "community" pages

[ukmo-ccbunney]

📚 Documentation

Reference: #4799

The "Iris in the Community" pages in the documentation are currently accessed via a tab from the home page.
Considering how important our community is to us, we should strive to make this more front-and-centre, perhaps considering where it might sit in the Diataxis model (#6511)

The community pages also need updating to highlight and showcase other libraries that Iris interacts with, for example (but not limited to) ncdata and geovista.