[docs-infra] Prevent 404 from demo to API pages

[alexfauquette]

The docs:api script is failing if an API page does not have a demonstration associated:
https://github.com/mui/material-ui/blob/59addf0b5519bd2f6f088031f7a71cc6ef0f40bf/packages/api-docs-builder/ApiBuilders/ComponentApiBuilder.ts#L745

But we miss the opposite check: make sure that all API pages listed in a demo exist.

This is not a straight forward modification because demos/api links are verified when creating an API page.
We should move this logic to get an overview of all the "demos to api" links to ensure they all exist.

Either by modify the docs:api such that componentsInfo.getDemos() coudl be replaced by an object { [project]: { [component]: demos }}

Such that by listing all the components with API page we can verify if some are missing

The other solution is to add the parsing of the components: .... in the script docs:link-check

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