docs(site): fix Lychee link check (drop cross-page fragments)

[tgoodwin]

Fixes the failing docs-site workflow so the docs can deploy.

The only failing step was the Lychee internal link check (strict mkdocs build is green). Lychee's --include-fragments can't resolve a cross-page link's directory URL to its index.html, so it reported two real anchors as missing. This drops the two #fragment parts (the links still land on the right page):

• extraction.md → cut-placement.md
• walkthrough.md → index.md

These were the only two cross-page fragment links in the site. Merging to main runs the deploy job → GitHub Pages.