docs: improve docs around proxy contract deployments

[kayagokalp]

Description

closes #7351

Improved documentation for the proxy contract feature in forc-deploy. The existing documentation was basic and lacked comprehensive details about the deployment process, configuration options,
and usage scenarios. This update provides:

• Detailed explanation of proxy contract purpose and benefits
• Complete configuration reference with field descriptions
• Step-by-step deployment workflows for both fresh and existing proxy scenarios
• Real deployment output examples
• Transaction details and storage slot handling information
• Important security and compatibility notes

The enhanced documentation helps users understand and effectively use the proxy feature for upgradeable contracts while maintaining consistency with the existing documentation style.A

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.