Fix and update switcher.json

[leehart]

The current documentation only goes up to version 13.0.0, but the latest release (at time of writing) is on v15.0.1.

[leehart]

@alimanfoo It looks like the version switcher has been broken by malformed JSON in your PR #729
e.g. https://malariagen.github.io/malariagen-data-python/latest/Ag3.html

I'll now attempt to fix with a new PR.

[leehart]

@alimanfoo Issue #578 is also potentially relevant, hence attempts to minimise the number of listed versions.