Debug new publish workflow

[hodgestar]

Fixes so far:

• Install dependencies for building plots.
• Separate images and plot folders.
• Remove trailing whitespace from op HTML templates.
• Remove call to create_index (bad copy and paste from qutip-tutorials).
• Remove unnecessary installation of wget.
• Publish the built site directly from website/_site.

[hodgestar]

The new publishing flow is working, so merging this PR now. We should check that it runs tonight after the nightly benchmarks.