Remove context provider APIs from advanced routing docs

[matthewp]

Description (required)

From https://github.com/withastro/roadmap/pulls it was decided to mark these APIs as internal for now, as we're not sure if we want integrations to extend the Astro global.

API still exists for internal features and could come back in the future.

Related issues & labels (optional)

withastro/astro#16980

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[ArmandPhilippot]

Thanks, LGTM!