Component library for classic-theme

## 💥 Proposal

- We should implement a component library for the classic theme.
- Those components would be reusable across the theme, and not coupled to a specific plugin.
- We could have a storybook for the theme
- We could do visual regression diffing using tools like Chromatic
- A user should be able to reuse such a component in custom pages

See also https://github.com/facebook/docusaurus/discussions/3064#discussioncomment-37801 => one usecase could to be give users the ability to reuse the docs sidebar on custom pages, if they want to create their own hardcoded sidebar in a custom page.

Some questions:
- Should it live in a separate package? so that all themes (including custom themes) can import it?
- Which components should be in this library?