Adds Theme Overview content block

[bertyhell]

https://meemoo.atlassian.net/browse/ARC-3814

Introduces a new content block for displaying themes in a structured and visually engaging way. This enables content managers to create curated theme overviews on content pages, enhancing the user experience for discovering related content.

The new block:

• Allows grouping themes under custom titles.
• Applies distinct colored background bands with dynamic shapes for each theme group.
• Implements a unique "braided snake" grid layout for theme tiles on larger screens, which gracefully adapts to mobile views.
• Integrates a Content Picker to easily select and manage themes within the admin panel.
• Links theme tiles directly to the search page for the selected theme.

Further changes include:

• A new ThemesService and associated hooks for fetching theme data.
• Updates to Content Block configuration maps and types to support the new block.
• Minor UI improvements for repeatable fields in the content block editor, including better styling for the delete button.
• Dependency updates for @viaa/avo2-types, @meemoo/react-components, and @viaa/avo2-components.

Relates to ARC-3814