This is the beginning of a page to collect common things users need to
do, in a format that they can copy & paste into their application code.
There's a lot more that could be done though.
Highlight all Slint code and don't do the preview on snippets where it
doesn't make sense.
Some snippets, such as the bare statement or expression snippets, are
now highlighted, but they are continued to be excluded from the doctest.