Theme gallery

## Theme gallery

We'd like to have multiple Docusaurus themes.

For now:
- only the classic theme is usable, which does not give many options to users.
- the bootstrap theme is a WIP but not really usable yet.
- the community would also appreciate a Tailwind theme: https://github.com/facebook/docusaurus/issues/2961

We agreed that those themes should all rather implement the same UX and work with docs, blog, pages...
This way, we can:
- Share some UI-related code across the 3 themes in a `@docusaurus/utils-client` package (edit: this is now `@docusaurus/theme-common`)
- Share the same integration test suite
- Share the same theme options, and validation/normalization
- Only implement styled components per theme

Related issues to read, as we already had theme-gallery related discussions with @fanny here:
- Cypress integration tests: https://github.com/facebook/docusaurus/pull/3468
- Sharing code between themes: https://github.com/facebook/docusaurus/pull/2714

The integration test suite will be handled in a separate issue https://github.com/facebook/docusaurus/issues/3523

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**IMPORTANT**: Algolia crawlers use DOM selectors to understand the hierarchy of the pages. We'll have to also make sure the default Algolia Docusaurus config works fine for all themes, so we should find a way to make the selectors "stable" across themes, in the same way they should be stable for tests. We'll probably add explicit "lvl0-lvl6" markers in each theme so that we don't break these selectors in the future.

See our current config: https://github.com/algolia/docsearch-configs/tree/master/configs/docusaurus-2.json