Add Playwright for end-to-end and visual regression testing

[jliermann]

This pull request is primarily focused on improving documentation formatting and readability, especially in the German-language VALIDATION_PROPOSAL.md file. It standardizes bullet point indentation, enhances list clarity, and applies minor structural improvements to the English documentation in docs/30_data/50_data_storage.mdx. No functional code changes are included.

Documentation formatting and structure:

• VALIDATION_PROPOSAL.md: Standardized bullet points and indentation throughout the document for better readability and consistency, including validation rules, workflow steps, and feature lists. [1] [2] [3] [4] [5] [6] [7]
• README.md: Adjusted bullet point formatting for the documentation links section.

Minor content and structural improvements:

• docs/30_data/50_data_storage.mdx: Improved section separation and added line breaks for clarity in workflow and backup recommendations, as well as in the sources section. [1] [2] [3]