feat(mantine): remove a lot of custom style to get a vanilla style

[GermainBergeron]

Proposed Changes

This PR is a first step to simplify and streamline the library by removing unnecessary customizations and aligning the library with a near-vanilla implementation of Mantine.

The UX team started using Pretine’s Figma library and it provides practically the same level of customization has Mantine does (via the Theme and component variables).

I divided the commits of this PR:

• Cleanup the style, remove a lot of unnecessary customization
• Add semantic components (ButtonPrimary, ButtonSecondary, ..., AlertWarning, AlertTip, ... etc) instead of having to rely on knowledge of the variant and colors
• Update the website to allow dark theme and changing the primary color

Potential Breaking Changes

A lot of minor changes to the style

Acceptance Criteria

• The proposed changes are covered by unit tests
• The potential breaking changes are clearly identified
• README.md is adjusted to reflect the proposed changes (if relevant)

[github-actions]

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