Add how-to guides section on the sidebar

[HuangJunye]

In order to add how-to guide example to Terra, we need to add a new section on the documentation side bar so that users can discover it.

[alt-shreya]

are you looking for contributors for this issue? I'd like to help!

[javabster]

closing this as it is covered by work in https://github.com/Qiskit/qiskit_sphinx_theme/issues/70