fix: update the docs for using a custom account

[moldy530]

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)
• Did you run lint (yarn lint:check) and fix any issues? (yarn lint:write)
• Did you follow the contribution guidelines?

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PR-Codex overview

Detailed summary

This PR updates the code in using-your-own.md file to show how to use your own smart account with the SmartAccountClient. It also includes an example of using LightSmartContractAccount. The notable changes include:

• Updated the code snippet for including the SmartContractAccount interface
• Added a code snippet for using your own account with SmartAccountClient

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