Upgrade to qiskit_sphinx_theme 1.12 and remove translations

[Eric-Arellano]

The 1.12 theme release is focused on stability, including CSS fixes. See https://github.com/Qiskit/qiskit_sphinx_theme/releases/tag/1.12.0rc1 for the changelog.

This PR removes the translations feature. We suspect it was incorrectly copied from qiskit-nature. We don't have any translations infrastructure set up for nature-pyscf, and it wouldn't work either way because these docs aren't deployed to qiskit.org/ecosystem.

This PR also:

• Fixes pyproject.toml to tell Python how to build the package. This is highly recommended to include.
• Enables -j auto with Sphinx to have parallelism. This should result in a faster docs build.
• Adds tox -e docs-clean

[coveralls]

Pull Request Test Coverage Report for Build 5070341324

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 69.697%

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Totals
Change from base Build 4992677623:	0.0%
Covered Lines:	69
Relevant Lines:	99

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💛 - Coveralls