feat: bring back last-updated metadata and add filtered git history

[rudigier]

What changed

• bring back Last updated metadata (date + author) for each page.
• Added expandable commit history for each page.
• Linked commit messages directly to their GitHub commit pages.
• Filtered out non-relevant commits via an exclusion list.
• Limited visible history to relevant commits from the last 5 years.
• Centralised repository URL in config.js (repoUrl).
• Refactored git history logic out of gulp/html.js into gulp/helpers/git-metadata.js.

Why

• Make page maintenance history transparent and useful for contributors.
• Reduce noise by hiding technical/maintenance-only commits.

[netlify]

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