doc: Use `Vue state management library` instead of `Vue Store` in docs

[awxiaoxian2020]

What problem is this solving

In Vuex docs, we use Vue state management library to describe it's function. But in Pinia, We use "Store" capitalized, which seems to imply that it is a proper noun, and it is. I personally think this description is unfriendly to beginners and translators.

Proposed solution

Use Vue state management library instead of Vue Store in docs.

Describe alternatives you've considered

No response

[posva]

They are different words, they were used in different contexts. Feel free to send your PR, maybe it does improve the docs if changed in some places