docs: add `autoUpdate` usage

To be included, seems it is not clear on docs how to use it and where/how to make the call to the virtual module (using import or dynamic import):

• add useRegisterSW fw variants on Prompt for update and Auto Update guide entries
• add autoUpdate entry on each fw with an small code snippet
• add autoUpdate with vue-router hint, maybe add small code snippet and add how to use it with SSR/SSG, with reference to vitesse template maybe will be sufficient

/cc @unpseudocomplique

[johannschopplich]

@userquin Not that important compared to your terrific work. 🙂

[johannschopplich]

@userquin Great update! I like it a lot, especially the explanation of the <ClientOnly> component at the end.

The only complaint I have is the wording of the intro section – maybe you could use a list instead of the multiple whens?