Furo: audit that it includes all relevant common files we export with theme

[Eric-Arellano]

Relates to #262.

We have several files that downstream repos may expect to have. For example, they may depend on some of these images:

├── images
│   ├── chevron-down-grey.svg
│   ├── chevron-right-purple.svg
│   ├── chevron-right-white.svg
│   ├── icon-close.svg
│   ├── icon-menu-dots-dark.svg
│   ├── launch.svg
│   ├── logo-facebook-dark.svg
│   ├── logo-icon.svg
│   ├── logo-twitter-dark.svg
│   ├── logo-youtube-dark.svg
│   ├── logo.png
│   ├── logo.svg
│   ├── logo_wordmark.svg
│   ├── qiskit-chevron-down.svg
│   ├── qiskit-chevron-up.svg
│   ├── search-icon.svg
│   └── view-page-source-icon.svg

To check for usage, we should grep for the file name. If it's not used by any HTML templates and also not used by their conf.py, e.g. for the below setting, we should be fine:

nbsphinx_thumbnails = {
    "sphinx_guide/notebook": "_static/images/logo.png",
}

[Eric-Arellano]

Confirmed we're good to go. logo.png is the only one used according to ripgrepping every file against every Qiskit project.