Validate anchors

[pepopowitz]

Update: with #4799 merged, we can now take advantage of the anchor validation that Docusaurus is already doing.

Our docusaurus configuration currently treats broken anchor's as warnings. We can add the setting onBrokenAnchors: "throw" to treat them as errors.

And then we'd have to fix several hundred broken anchors (according to the most recent build) 😅 .

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Old description

We are currently using untitaker/hyperlink to check internal links. It is only validating that a target page exists.

We should consider validating anchors, too. There is a --check-anchors option to enable this.

Turning this flag on might open Pandora's box of invalid links, so be prepared for this work to include fixing a lot of existing links.