Revert adding more padding to Furo table of contents

[Eric-Arellano]

I thought that this was a good idea because we intentionally make the left sidebar overall bigger.

But I realized when looking at the Qiskit Experiments docs that we have a lot of API page titles, and those tend to have long names.

So, it's better to fully use the screen real estate and use Furo's default padding rules, which IMO are sufficient.

We were also diverging from IBM Quantum, which uses 16 pixels of padding.

Finally, this avoids weirdness with the right chevrons not having enough padding. Closes #411.

[coruscating]

Thanks, the Experiments docs look better now :)

[javabster]

still a bit unsure about the padding on the right of the chevrons, but let's keep it like this for now and we can revisit it later if it becomes an issue 🚀