Add Technical Engagement Right Sidebar and Data Refactor

[rockoder]

This PR implements a technical engagement right sidebar to utilize empty space on larger screens (>= 1280px).

Key changes:

1. Data-Driven Architecture: Converted static Markdown tables for Reading, Writing, and Video lists into structured YAML data in _data/. This enables dynamic extraction of "latest" items for the sidebar.
2. Knowledge Graph (Idea Added first baeldung article #1): Implemented "Related Concepts" in the sidebar, which dynamically finds and links posts sharing common tags with the current article.
3. Current Research (Idea All my Baeldung articles published so far #2): Implemented a "Currently Reading" section pulling from the structured book data, including a monetization link.
4. Recently Explored: Added a section for latest posts to keep readers engaged with recent content.
5. Responsive Styling: The sidebar is fixed and follows the site's minimalist, technical aesthetic. It is hidden on mobile and tablet views to preserve focus.
6. Hygiene: Cleaned up temporary build files and reverted a permalink change to ensure zero regressions.

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PR created automatically by Jules for task 9197514160781108108 started by @rockoder

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