[Feature] Add "Scroll to Top" button to website pages

### What problem does this feature solve?

Yes. When navigating through long pages on the ECharts website (such as the Homepage, lengthy Documentation sections, or the Examples gallery), it is cumbersome to manually scroll back to the top of the page. This negatively impacts the user experience, especially on mobile devices or when browsing extensive API references.Many modern documentation sites (e.g., those built with Docusaurus or VitePress) include this feature by default. Adding it to the ECharts site would align it with modern web standards and improve navigation efficiency.

### What does the proposed API look like?

Add a global ScrollToTop component