Actually (re-)write introductory and conceptual documentation

[axch]

Presumably this will have a lot of overlap with the paper.

Components:

• [ ] Purpose and summary.
• [ ] Advertising (it's fast, it's easy to install and configure).
• [ ] Installation instructions.
• [ ] Usage examples (Issue #23).
• [x] What equations does the model solve?
• [ ] Numerical description: name of integration method, grid type, any other considerations.
• [ ] Maybe pointer to developer documentation, produced by Doxygen.
• [ ] Maybe pointer to auto-built test/example results, per #65.
• [ ] Comment that there is a separate Fortran program under all the Python, which can be invoked standalone profitably enough.

[edoddridge]

Definitely a lot of overlap, but also some quite different content. For example, I don't expect the paper to describe the installation procedure in detail.

[axch]

The ideal installation instructions are "pip install aronnax", which, to be true, calls for #87 being done.