Enhance documentation for blog post creation and content structure

[danielmarv]

This pull request significantly improves and clarifies the documentation around adding blog posts and using front matter in the content system. It introduces a dedicated guide for blog post creation, updates front matter standards to distinguish between standard pages and posts, and ensures that routing, localization, and metadata conventions are clear for contributors. The changes also update checklists and cross-references throughout the documentation to reflect these new standards.

Major documentation improvements:

• Added a new guide, docs/07-adding-blog-post.md, detailing the workflow for adding blog posts, required front matter fields, file naming, routing, image handling, and common pitfalls.
• Updated the main documentation hub (README.md) and reading path to reference the new blog post guide, and clarified when to use it versus the standard pages guide [1] [2] [3] [4] [5] [6].

Front matter and content standards:

• Revised docs/02-content-folder.md to clearly separate required front matter fields for standard pages vs. blog posts, provide field-specific notes, and supply updated YAML examples for each [1] [2].
• Added explicit instructions for blog post front matter, including new fields like excerpt, categories, preview_image, showInBlog, and outdated [1] [2].

Localization and routing conventions:

• Clarified URL and routing rules for both standard pages and blog posts, emphasizing the /posts/... pattern (not /blog/...), and how filenames map to URLs in both English and German [1] [2] [3] [4].
• Updated localization parity requirements and checklists to ensure blog posts and standard pages are handled appropriately for both languages [1] [2].

Workflow and checklist updates:

• Updated pre-submission checklists and best practices to reflect new blog post requirements and validation steps, including front matter fields and localization expectations.
• Clarified the distinction between guides for adding standard pages and blog posts in docs/03-adding-pages.md and other locations.

These changes make it much easier for contributors to add new blog posts correctly, follow best practices for metadata and localization, and avoid common mistakes.