docs: update account usage in core package

[howydev]

Pull Request Checklist

• Did you add new tests and confirm existing tests pass? (yarn test)
• Did you update relevant docs? (docs are found in the site folder, and guidelines for updating/adding docs can be found in the contribution guide)
• Do your commits follow the Conventional Commits standard?
• Does your PR title also follow the Conventional Commits standard?
• If you have a breaking change, is it correctly reflected in your commit message? (e.g. feat!: breaking change)
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PR-Codex overview

This PR updates the documentation for sending user operations by introducing the SmartAccountClient and changing the underlying Smart Contract type from LightAccount to ModularAccountV2.

Detailed summary

• Added information on creating a SmartAccountClient using the createModularAccountV2Client function.
• Introduced the use of the sendUserOperation function for sending user operations.
• Changed the underlying Smart Contract type from LightAccount to ModularAccountV2 in examples.

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Name	Status	Preview	Comments	Updated (UTC)
aa-sdk-site	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Mar 11, 2025 6:01pm
aa-sdk-ui-demo	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Mar 11, 2025 6:01pm

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[jakehobbs]

lgtm