Add docs for flexible assets from LaTeX document

[alexdewar]

As we are planning to implement flexible assets for this engagement (#360), we should add the relevant documentation from the LaTeX original. While we're at it, it would be worth proof-reading to check that it makes sense as an approach. Then we can double-check with Adam that this is what he wants before we start it.

Available here (private link) in dispatch.tex: https://www.overleaf.com/project/682d92581b6f0e83d44ac53d