docs: clarify %%bigquery` magics and fix broken link

[tswast]

The link at https://googleapis.dev/python/bigquery/latest/usage/index.html
was broken. Also, I was having trouble remembering what I needed to do to
enable the %%bigquery extension, so I put that prerequisite right at the
top of the docs.

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[plamut]

LGTM. The link in the docs now works and explaining how to load the extension can be useful, too.