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ARROW-5746: [Website] Move website source out of apache/arrow

Companion to https://github.com/apache/arrow-site/pull/9

In addition to `rm -rf site`, this patch edits a few scripts that reference the site directory. The least invasive approach I found was to add `site/` to `.gitignore` and recommend that folks can clone `apache/arrow-site` there, at which point existing scripts should mostly just work.

There were also a few misspellings that incorrectly matched `grep site` when I was searching for those places to edit.

Closes #5015 from nealrichardson/move-website-source and squashes the following commits:

c23a26951 <Neal Richardson> Add this dockerignore back in case people use the site dir locally
e811f94ba <Neal Richardson> Resolve TODOs by recommending cloning arrow-site into the (now gitignored) site directory
96ba26811 <Neal Richardson> Remove website source, note a few TODOs for doc building; fix some typos that 'grep site' shouldn't have hit

Authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>