Introduce Design Tokens

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### Description

With Penpot 2.6.0 design tokens are introduced. Design tokens are used thoroughly in Material 3, and could be used by the plugin to leverage the theme generation with not only colors, but also dimensions, border radius, opacities, rotations, sizings, spacings, strokes and eventually typography (not included in current RC).

### Acceptance Criteria

- The plugin generates themes and sets for the design tokens
- The plugin populates the sets with the correct values and aliases
- The documentation pages are updated
- The user can still add multiple themes and switch between them
- The generated tokens support light / dark theme with contrast levels
- The generated tokens use the design token names from M3
- "Switching between themes" is intuititve and self-explanatory
- Obsolete functionality in the plugin is removed (e.g. theme switching)

### Additional Information

This issue does not include the generation of the components, but the resolution may cover some of the component tokens.

Start by reviewing the plugin's existing theme-generation and documentation entry points, then compare them with Penpot 2.6.0 design tokens and Material 3 token names. The work is done when themes and sets contain the required values and aliases, support light/dark contrast levels and switching, documentation is updated, and obsolete theme-switching functionality is removed.

Written by the indexing model from the issue text.