feat: add check for summary and update contribution guide

[f-hollow]

Description

This PR updates the following:

• Adds a check for the presence of PR summaries
• Adds the blog archetypes file (template for blog articles)
• Improves the contribution guide
• Improves the summary CSS style
• In the CI job where summaries are added, update the logic for link checking to trigger only if README.md or content/ were updated

The blog archetype file -- the template for blog articles -- now has guidelines. Some of them were moved from the Contribution guide, others were written anew.

Related

• PR Feat/style and publish front matter summary in article #402
• PR Ci/add articles in yyyy/mm folder #472

Testing

Tested the CI checks in this PR and in !16.

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.

[f-hollow]

@pedrominatel @FBEZ PTAL

Let me know if anything is not clear enough, I will add more descriptions.

[pedrominatel]

Thank you @f-hollow!
Just 2 minor comments. LGTM.

[f-hollow]

@pedrominatel PTAL again and, if no comments, feel free to merge it.

[pedrominatel]

@pedrominatel PTAL again and, if no comments, feel free to merge it.

Thanks @f-hollow, LGTM.