[ DESIGN ] Update API Docs Styling

[Eric-Arellano]

Part of #479. This issue is focused on the visual design of the API docs.

Right now, API docs are not using design elements to accurately reflect information hierarchy. For example, no colors or indentation.

This is an example class:

Some methods:

A standalone function, but note that it starts after the Return type QasmQobj | PulseQobj text:

Compare to qiskit.org

These screenshots come from our repository for the Qiskit Sphinx Theme, the files starting with api-docs-: https://github.com/Qiskit/qiskit_sphinx_theme/tree/main/tests/js/tests.js-snapshots

A class with methods:

A module page showing a standalone function

[gracelindsell]

@Eric-Arellano Here are the breakpoints for the API Docs styling. To the right please find a version without indentation for initial implementation

[Eric-Arellano]

Thanks! Implementation is tracked by #735 and #736, where I also put screenshots.