Converting to django measurement

[maxfenv]

This PR chiefly adds some documentation about converting from a plain old FloatField to a MeasurementField. I had written this up in an internal wiki article, but realised that it's much better off here.

While doing so, I also:

• Converted some print statements in the documentation to python3
• Added a numeric prefix to documentation files in docs/topics so that the order in the table of contents is sensible
• Marked one of the docs sources as not executable (!)