Docs: add wallet plugin

We document other plugins: https://www.solanakit.com/docs/plugins/available-plugins

But haven't got `kit-plugin-wallet` yet.

We will probably also want to add the plugin's react hooks to the react section, following whatever pattern we decide to use for other plugin hooks (like the instruction plan ones).

Start from the existing plugin docs at https://www.solanakit.com/docs/plugins/available-plugins and find the source pages for other documented plugins. Add equivalent documentation for `kit-plugin-wallet`; also inspect the React section and existing plugin hook docs such as the instruction plan ones before deciding whether wallet hooks fit there. Done means the wallet plugin appears with the other available plugins and any matching React hook docs follow the established pattern.

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