Epic: Add Ecosystem Theme Variant

[javabster]

Background

We need to differentiate the branding of “core” Qiskit (Terra) from the “ecosystem”, e.g. Qiskit Nature and third-party extensions. For example, their docs need a different logo with one saying “Ecosystem”.

Goal

When a user comes to the documentation page for an ecosystem project (e.g. qiskit.org/documentation/nature) they should be able to see from the style of the website that the project is affiliated with Qiskit but is clearly part of the ecosystem, not Qiskit itself.

[Eric-Arellano]

https://github.com/Qiskit/qiskit_sphinx_theme/pull/229 adds the infrastructure for this: https://github.com/Qiskit/qiskit_sphinx_theme/pull/229

What remains is figuring out all the above design differentiators, and then implementing them in qiskit_sphinx_theme repo.

[Eric-Arellano]

The Ecosystem Theme is now being used successfully by 5 projects:

• https://qiskit.github.io/qiskit_sphinx_theme/
• https://qiskit-extensions.github.io/circuit-knitting-toolbox/
• https://qiskit-extensions.github.io/quantum-serverless/
• https://qiskit.org/ecosystem/ibm-provider/
• https://icekhan13.github.io/purplecaffeine/

Those last two show off custom logos.

We will probably still have future improvements to make, but we can close this issue out 🙌 Thanks @eddybrando @techtolentino @javabster for the design input back in May! And @frankharkins @coruscating for the input and code reviews