Change S3 bucket from old dvc-share to new dvc-public/{prefix}

[jorgeorpinel]

We decided to migrate from an old S3 bucket dvc-share to use certain prefixes (folders) in a new dvc-public bucket for use in documentation. The prefixes identified now are:

• code: For example code archives
• data: For sample datasets
• remote: For using as an S3 remote (read-only in examples)

The main use for the bucket is to wget (download) files or use as DVC remote (read-only) in the examples throughout command references and Get Started section (as well as in https://dvc.org/doc/commands-reference/get/url) but there are other uses.

• The engine has some hardcoded HTTP 301 redirects pointing to the old bucket. We need to change those.
• All the examples that use either redirected dvc.org URLS or the old bucket directly need to be updated and tested.

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Spanned from #487

[jorgeorpinel]

Also found that

• http://man.dvc.org/get-url gets redirected to https://dvc.org/doc/commands-reference/get/url (404 Not Found)