Add Vue.version and app.version to the API reference

[skirtles-code]

Closes #997.

This PR documents Vue.version and app.version in the API reference. These are two different ways to access the current Vue version.

In Vue 2, we just had Vue.version: https://vuejs.org/v2/api/#Vue-version. When interacting with a global Vue object, e.g. for CDN usage, the equivalent in Vue 3 would be exactly the same, Vue.version.

For plugins, the first argument passed to install was previously Vue. It's now app, which also exposes a version property. This allows plugins to check the version using the version property of that first argument.

I borrowed much of the wording and example code from the Vue 2 documentation.