fix: resolve function scope and component render issues in site/App.js

[Kozak809]

1. Summary of changes: Refactored the Readme component in site/src/App.js to directly render ReactMarkdown instead of using ReactDOM.render within useEffect. Corrected a scoping issue where the proposal function was used before it was defined in the App component by utilizing useCallback and useMemo. Cleaned up unused imports in site/src/App.js and removed dead commented-out code.
2. Rationale behind the implementation: The previous implementation used ReactDOM.render inside useEffect, which is an anti-pattern in React that can cause memory leaks and improper rendering. Additionally, the original code had a functional scoping bug where the proposal function was defined using a function expression after it was referenced in the columns array configuration, which would lead to it being undefined at initialization. The refactor ensures correct component lifecycle management and fixes the initialization bug.
3. Technical impact analysis: These changes improve the robustness and maintainability of the site application, fix the broken proposal click handler, eliminate potential memory leaks, and reduce the bundle size by removing unused antd imports and dead code.