[Docs] Guide on hydrate for SSR

[Jamiewarb]

When the store values differ on Server and Client, such as when using useStorage from Vueuse, the client store needs to be hydrated manually.

There's a section on that within the API reference part of the docs here: https://pinia.vuejs.org/api/interfaces/pinia.DefineStoreOptionsInPlugin.html#hydrate

However, I wanted to propose adding a small section on it under state within the Guide part of the docs.

I've opened this issue to ask for your thoughts on that @posva before I submit a PR with potential documentation for it.

Let me know what you think :)

[Jamiewarb]

Aggregated with other docs recommendations in #829