[blocked] Terra's How To and Explanations docs not hooked up

[Eric-Arellano]

We need to clone the docs and add them to index.rst. This will be easy to do with an upcoming refactor I'm making.

Update: See Qiskit/qiskit#9716 (comment). We need to cherry-pick the docs to 0.24 to do this, or have changed the docs build to 0.25+.