Speed up dev build of docs

[Eric-Arellano]

The Furo docs theme we now use is naively slower than our old Pytorch theme (Qiskit/qiskit_sphinx_theme#328): the docs build wall-time goes from 13.75 minutes to 40 minutes.

So, this PR distinguishes between prod builds vs. dev/CI builds. CI builds only take 15 minutes, a 1.25 minute slowdown from Pytorch. The slowdown comes from viewcode (14 minutes) and including the API docs in the left sidebar (11 minutes).

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Eric-Arellano
• @javabster
• @Qiskit/terra-core

[CLAassistant]

All committers have signed the CLA.

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[coveralls]

Pull Request Test Coverage Report for Build 5860751626

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 19 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.01%) to 87.242%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	1	90.89%
crates/qasm2/src/parse.rs	18	96.67%

Totals
Change from base Build 5860553430:	-0.01%
Covered Lines:	74282
Relevant Lines:	85145

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💛 - Coveralls

[mtreinish]

This LGTM, I also took a look at the dev docs build locally from the ci artifacts and it seems like a reasonable tradeoff for speed vs a "complete" build. The lack of the full toctree for the sidebar and the viewcode extension don't impact the test coverage of docs builds in a meaningful way. I think the only thing missing now is to just update this to address the merge conflicts and also update the new deploy jobs to set DOCS_PROD_BUILD so we're building the full docs for publishing..

[Eric-Arellano]

Doesn't matter much, but to be consistent with rest of the file.

[mtreinish]

LGTM, thanks for the quick update.