Set v1.2 as the "latest" release on the documentation website

[gaurav]

We released v1.2 of the Python data classes just before the Christmas break (see https://pypi.org/project/crdch-model/1.2/). We've published this to the documentation site (https://cancerdhc.github.io/ccdhmodel/v1.2/). However, we wanted to check with the DMH team before assigning this to be the latest version of the site (i.e. https://cancerdhc.github.io/ccdhmodel/); since the project was terminated before we could do this, we have left the v1.1 release as the latest release. There should be very few and relatively minor changes between the data model described be v1.1 and v1.2.