[Website] Add overview page

[asfimport]

Kudu has a good model, linked prominently from the home page: https://kudu.apache.org/overview.html

Should go into some more explanation of why Arrow was created, what the vision of the project is, and what's in the project currently. Kind of an expanded, more technical version of the high-level bullets on the (new) home page.

Reporter: Neal Richardson / @nealrichardson
Assignee: Neal Richardson / @nealrichardson

_{Note: This issue was originally created as ARROW-9166. Please see the migration documentation for further details.}