Crate-docs-theme

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- Fix URLs to GitHub feedback, removed hard-coded ``https://cratedb.com/``

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- Fix downstream behavior on "not versioned" projects like CrateDB Guide.

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- Remove Google Search and implement Algolia docsearch
- Remove `search.html` and `genindex.html` from `site.xml`

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- Cleanups: Remove configurations for ``cloud_{howtos,reference,tutorials}``
- Fix computing URLs for rel="canonical"

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- Fix compatibility issue with Python 3.8, by updating to
sphinx-design-elements 0.3.1

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- Fix missing CI colors to align with the main website
- Update icons with the corresponding colors
- Introduce a secondary color for improved accessibility
- Small improvement for the version/feedback dropdown on mobile
- Remove intersphinx mappings to ``crate-howtos`` and ``crate-tutorials``,
they have been converged into ``cratedb-guide``.
- Update to sphinx-design-elements 0.3.0