Broken references to top-level API reference path

[marc1404]

The documentation contains references to the API reference starting from the root: https://gardener.cloud/api-reference/. Currently, there is no content published at this path, and it leads to an HTTP 404 Not Found.

Other pages refer to the API reference at the root path, which indicates that we overlooked this during the migration from Hugo to VitePress.

Example reference:
https://gardener.cloud/docs/extensions/infrastructure-extensions/gardener-extension-provider-aws/#learn-more

[marc1404]

Related to: #726

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