Make Mesh documentation more obvious

[stephenworsley]

📚 Documentation

Currently the mesh documentation exists in "Further Topics" of the user guide. This isn't obvious to everyone at a first glance of the documentation and, from experience, often has to be pointed out and highlighted to people. Since Meshes are now out of experimental and are being more frequently used, it might be time to promote the Mesh documentation to a more obvious part of the documentation, e.g. the user guide itself (between Cube maths and Citing Iris).