docs: address review feedback on landing page

- Fix html_title frontmatter that was leaking as literal text on the page
- Promote 'Apache DataFusion Comet' to the hero H1 for proper brand prominence
- Restore 'Use Commodity Hardware' section using verbatim source copy
- Fold 'Tight Integration with Apache DataFusion' into Architecture intro
- Mention Comet in the terminal comment
- Mark decorative terminal traffic-light dots aria-hidden
- Remove redundant alt text on footer logo

Author: pranamya123