Minimise content on main documentation page

[ehogan]

There are a few outstanding comments from #3914 that shouldn't hold up merging, so I'm moving them to a new issue (this one!). Both suggestions are related to making the main page less busy and making information readily available regardless of which page in the documentation the reader is on.

• Remove the "Support and contact" section and instead include the information on every page, e.g. in the footer, or the secondary sidebar (under the "Show Source" link) (add example code to this issue to do this).
• Remove the "Learning resources" section and instead include the information on a new page, then replace the "Tutorial" link in the header / navigation bar at the top with a link to this page. We will have multiple tutorials in the future, so linking to a page of "Learning resources" rather than a direct link to the current tutorial would be beneficial.