The three Self-Paced Tutorial sidebar entries pointed at .md files
that were either outside the docs/ Quarto project (../tutorial/*.md)
or had a .md href Quarto won't serve as a page (laws-explained.md).
Bring all three under docs/ as .qmd so Quarto renders them:
- copy tutorial/{exercises,cheatsheet}.md -> docs/tutorial/{...}.qmd
- rename docs/laws-explained.md -> docs/laws-explained.qmd
- update sidebar hrefs in config/_quarto-html.yml
