Fix broken link checker workflow

[alexdewar]

Description

The link checker workflow is currently not working, which is causing other open PRs to fail CI checks.

There are a few reasons:

• A couple of broken links in docs/model/investment.md
• We're getting really weird 404 errors for parts of the docs for the erased-serde crate that are linked to from our API docs, even though it should be generated automatically. Not sure what the problem is, but I've just ignored these for now.
• A new weird error in the book/404.html file, which is also autogenerated, so I'm just ignoring the file now

The link checker workflow doesn't actually run on pushes to main. We are checking that the docs build ok, as part of the docs deployment workflow, but maybe we should also check links here too.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc
• Update release notes for the latest release if this PR adds a new feature or fixes a bug
  present in the previous release

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.52%. Comparing base (3d4b209) to head (237de2b).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1147   +/-   ##
=======================================
  Coverage   87.52%   87.52%           
=======================================
  Files          55       55           
  Lines        7626     7626           
  Branches     7626     7626           
=======================================
  Hits         6675     6675           
  Misses        649      649           
  Partials      302      302           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[tsmbland]

Why do we have to use a URL here rather than an internal link?

[Aurashk]

thanks!