docs: facelift the docs

[mistercrunch]

SUMMARY

40k+ people visited supserset.apache.org last month. It looks a bit like it's been put together by a backend engineer, so a few touchups go a long way. Improvements:

• using the same antd theme as on Superset (though that affects a single button at this time, but using antd components should align more with Superset's theme in the future)
• remove the footer that was all funky-looking, moving the links to the navbar
• use a GitHub logo for the GitHub link on navbar-right
• use Inter (same font as Superset)
• make screenshots width more bearable (80% -> 800px)
• hide fugly buttons on search input box
• remove all external links icons (surprisingly hard to get docusaurus to do this)

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

[rusackas]

Clicked around locally, and lookin' good to me too! Thanks for sweeping up! ✨