fix(ui): limit Markdown rendering on RichInput (#6061)

- Added a new `useMarkdown` prop to RichInput for toggling between Markdown and HTML content.
- Updated the `extensions` function to conditionally configure link support based on the `useMarkdown` prop.
- Modified the content update logic to handle Markdown conversion when `useMarkdown` is enabled.
- Adjusted initial content loading to respect the `useMarkdown` setting for proper content type detection.