Document gotchas related to authoring DSP live examples

[tomivirkki]

There are a couple of essential technical details in DSP that must be considered while working with live examples, and they haven't yet been properly addressed in the DSP documentation.

1. When adding a live Flow example, an associated TypeScript file needs to be created to import the necessary web components and other client-side modules. In the case of Vaadin documentation, this is typically covered by the Lit and React examples which take care of the web component imports also required by the Flow examples.
   If an example group only has a Flow example, a separate TS file with the imports must be added to the group with preimport, hidden tags. Here's one such example group on Vaadin documentation.

2. If the custom theme includes an html CSS selector, it needs to be extended to html, :host() since live examples on the documentation are rendered as web components and an html tag doesn't exist inside a shadow root. This is already documented here, but it should be clearly referenced in the DSP documentation.