Update Nuxt module README

[toto6038]

This PR rewrites README for @pinia/nuxt, which is also displayed at module page. Specifically, the changes include:

• remove mention of Nuxt 2 and Nuxt Bridge
• add automatic installation guide
• describe module configuration and mention auto import
• add note for Nuxt 4's new directory structure

[netlify]

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[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@pinia/nuxt@3013

npm i https://pkg.pr.new/pinia@3013

npm i https://pkg.pr.new/@pinia/testing@3013

commit: e0a92c4

[posva]

Thanks!