fix(tutorial): prevent Typescript error + tiny data reordering

[ArmandPhilippot]

Description (required)

• Adds missing slashes to links in Unit 3.3: we had them in Unit 3.1 and they are back in Unit 5.3.
• Fixes a Typescript error in Unit 3.4: the script in index.astro requires optional chaining and the setAttribute value should be a string.
  This is flagged by Typescript because the default preset is strict and we don't tell people to switch to basics in Unit 1.2.
  Note that this is no longer an issue once moved to scripts/menu.js but I kept the same code to avoid any confusion or extra wording.
• Reorders MarkdownPostLayout.astro frontmatter data in Unit 5.3: Unit 4-2 and Unit 4-3 use a different order, we had the frontmatter.description between the date and the author.
• Reorders frontmatter data in Unit 6.3: Unit 2.4 used a different order

Except the Typescript error, this is mostly nits... But this could be confusing for beginners when they check if their files are correct!

Related issues & labels (optional)

• Suggested label: code snippet update
• Related: This is part of my blog-tutorial-demo branches updates:
  • fix(complete): update deps, clean up & fix mismatches with docs blog-tutorial-demo#45
  • fix(content-collections): update deps, accessibility, clean up & mismatches blog-tutorial-demo#46

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[sarah11918]

Thanks! All looks great!