Doc ref sidebar

This PR reduces the `maxdepth` of the Sphinx generated sidebar for the opening page of the Iris reference documentation.

This sidebar was rather long and occupied a significant proportion of screen real-estate. Also in certain versions of Firefox, lower sidebar links i.e. `iris.palette`, `iris.pandas`, `iris.plot`, `iris.proxy` etc failed to render correctly and did not allow users to follow the link to the content.