Reduce the reliance on the global Vue in API Reference examples

[skirtles-code]

This is the latest stage of #773, attempting to make the examples less specific to a particular build.

This PR attempts to remove the Vue. prefix from all of the examples in the API Reference. In most cases the examples are only using createApp, so the prefix is simply dropped. For examples that use other parts of the global API, where it may not be immediately clear to some readers that a particular function comes from the global API, I've switched the examples to use destructuring instead.

The Global API page itself now has an introduction explaining the two different ways to access the functions listed on that page.

There are some examples that explicitly use import to pull in parts of the global API. I haven't changed any of those here, though in a handful of cases that seems just as unnecessary as referencing the global Vue.