Enable dead link detection mechanisms

[marc1404]

We disabled the dead link detection of VitePress during the migration effort:

documentation/.vitepress/config.mts

Line 40 in 5389b07

ignoreDeadLinks: true, //ToDo enable after migration

We should enable this feature to catch at least some of the broken references during build (this would also show up during PR checks).
As it seems, the detection does not work during the development cycle of VitePress:
vuejs/vitepress#4419

However, checking this during build time would be a good first step.

Additionally, Docforge also has a dead link detection mechanism:
https://github.com/gardener/documentation/blob/master/.docforge/config.yaml#L29

Let's look into enabling this one as well.

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