Reorder top-level documentation headings

[gassmoeller]

This PR reorders all of the top-level sections of the documentation according to the documentation plan (https://docs.google.com/document/d/1qxnbEpvC8UMeh98dMDcbBtHp8ylC9YM4RXpJREB1Dms/edit?usp=sharing).

Some pages still need text, but overall the new layout is ready to merge. I decided to make one change in that I put "Accessing and sharing data" as a sub point under "Research enabled by Rayleigh", because the datasets are really also research results we want to showcase (plus the structure of the headings and subheadings in the left sidebar seemed to make more sense to me). If you want me to stick to the original order let me know. I will post where to check the new documentation once readthedocs is done building it.

[gassmoeller]

You can see the new top-level documentation structure here for review.

[feathern]

Looks good. I see that the read-the-docs build test seems to be hung. I'm going to go ahead and accept this.

[gassmoeller]

Thanks. Readthedocs has a limited number of virtual machines that will build our documentation. If we open too many pull requests they will take a while to catch up. By now it is done.