fix(Walkthrough): fix link, add path to link-checker

Description

Repairs the broken link @ghavenga reported in https://gitlab.slack.com/archives/C013QJ9NEPL/p1744318002763989.

Lychee can't catch this with the current configuration. Two reasons:

• walkthroughs/**/*.md wasn't being linted
• it's an external link, which wouldn't be linted in --offline mode (sigh)

I can add the path to the YAML file, but external links still won't be checked.

Related Issues

Resolves #[issue_number]

How has this been tested?

• If src/browser or src/common has been modified, please consider interoperability with the Web IDE. See Running the Extension in WebIDE.
• Consider an end-to-end test for significant new features that aren't covered by integration tests.

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes