Doctests are broken, should be run on CI

E.g. in https://github.com/PainterQubits/Unitful.jl/blob/master/docs/src/conversion.md#unit-cancellation, there's a double `julia>` prompt, which breaks the test. Looks like doctests are not running on CI otherwise this probably would have been caught.

Start with docs/src/conversion.md at the unit-cancellation section and inspect how the project's documentation examples are currently processed. Check the existing CI configuration for any doctest step, then run the documentation tests locally. Done means the duplicated prompt is corrected and CI reliably executes the doctests so similar errors are caught.

Written by the indexing model from the issue text.