Enhance Readme with Designer's Collaboration Guide

[ulusoyca]

Description

This PR introduces a new section to the README.md titled "Designer's Collaboration Guide." The goal is to streamline the collaboration process between designers and developers by providing a detailed guide and a Figma file link. This guide includes design specifications, modal types and states, and motion animation specs tailored for the WoltModalSheet package.

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes tests for all changed/updated/fixed behaviors.
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• The package compiles with the minimum Flutter version stated in the pubspec.yaml

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

[github-actions]

Visit the preview URL for this PR (updated for commit aebc153):

• https://coffeemakerexample--pr168-introduce-figma-spec-n4a39iti.web.app
• https://playgroundwoltexample--pr168-introduce-figma-spec-1hvz4x6q.web.app
• https://playgroundnavigator2woltexample--pr168-introduce-figma-6ikbyygk.web.app

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