fix: Add required ASF policy links to footer

[rbowen]

Adds Foundation, Events, License, Security, Sponsorship, Thanks, and Privacy links per ASF website policy.

Checker: https://whimsy.apache.org/site/

[github-actions]

Preview URL: https://rbowen.github.io/arrow-site

If the preview URL doesn't work, you may forget to configure your fork repository for preview.
See https://github.com/apache/arrow-site/blob/main/README.md#forks how to configure.

[alamb]

Here is what the page looks like on main (before this PR)

Here is what it looks like with this PR

With @kou 's proposal from #785 (comment)

In my mind, @kou 's is a clear improvement so I will apply it

[alamb]

Ok, I cleaned up a conflict, and rechecked this locally

I also verified all the links went to the right place.

THanks again @rbowen and @kou

[kou]

Thanks for completing this!