.github: Add cpp_docs job to current gcc5 workflow (#64044)

.github: Add cpp_docs job to current gcc5 workflow (#64044)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64044

Adds the cpp_docs job to the current workflow, also modifies the scripts
surrounding building docs so that they can be powered through
environment variables with sane defaults rather than having to have
passed arguments.

Ideally should not break current jobs running in circleci but those
should eventually be turned off anyways.

Coincides with work from:
* https://github.com/seemethere/upload-artifact-s3/pull/1
* https://github.com/seemethere/upload-artifact-s3/pull/2

Signed-off-by: Eli Uriegas <eliuriegas@fb.com>
cc ezyang seemethere malfet walterddr lg20987 pytorch/pytorch-dev-infra

Test Plan: Imported from OSS

Reviewed By: malfet

Differential Revision: D30610010

Pulled By: seemethere

fbshipit-source-id: f67adeb1bd422bb9e24e0f1ec0098cf9c648f283