Docsite refactoring (thoughts & opinions welcome!)

[Sam Redai <sa...@tabular.io>]

Hey everyone,

Just wanted to highlight PR #3378
<https://github.com/apache/iceberg/pull/3378> that I opened this morning.
It's a pretty big change to the current apache iceberg docs site. If you
have python and mkdocs installed locally you can clone and serve the site
to take a look. There's a single screenshot of the home page in the PR but
you can only tell so much from that.

We have some more changes slotted for the content once the new site is
live, but I'd love to get as much feedback in this PR on the general design
and direction.

Thanks!
-Sam