Remove the Azure MinIO gateway page and its example

What does this MR do?

The Azure MinIO gateway page documents a MinIO mode that no longer exists, using an image that can no longer be pulled. MinIO removed the Azure gateway in April 2022 and the rest of gateway mode in October 2022, and Docker Hub denies pulls of minio/minio, the image in the page's az webapp create step.

• Replaces azure-minio-gateway.md with a redirect to the external object storage index, which covers direct Azure Blob Storage support. The stub follows the redirect procedure and expires on 15 January 2027.
• Drops the two links to the page from _index.md: the sentence at line 40 that pointed readers to it, and the "like Azure using MinIO" aside at line 187.
• Replaces the s3cmd "On Azure Storage" bullet at _index.md:315 with a pointer to the native azure backup backend, which the backup and restore docs already describe. The bullet configured the gateway host.
• Deletes examples/objectstorage/rails.azure.yaml, which pointed Rails at the gateway host, and its entry in the index. rails.azurerm.yaml stays as the Azure example, and nothing outside the translated copies references the deleted file.
• Trims the two remaining gateway warnings in _index.md: the registry note at line 43 keeps its registry.azure.yaml pointer, and the S3 aside at line 94 now reads "(S3 compatible storages)".

The translated copies under doc-locale/ are left to the localization team, who alone can change them (documentation guidelines). Until they resync, the translated index still links to its copy of this page and to the deleted example.

The page's navigation entry in docs-gitlab-com (data/en-us/navigation.yaml) comes out in the companion MR gitlab-org/technical-writing/docs-gitlab-com!2767 (merged), as the redirect procedure asks. The ja-jp entry (data/ja-jp/navigation.yaml:4531-4532) stays with the localization team.

If you would rather keep the page, a warning admonition at the top would do instead. That is an easy swap.

An AI coding assistant wrote this change. It checked each claim against its source: the MinIO release notes and pull request, Docker Hub, the redirect procedure, the backup and restore docs and the docs-gitlab-com navigation files. It searched this repository for references. On a test cluster, it rendered and installed the chart from master and from this branch, with no difference in the rendered manifests, and ran Vale, markdownlint, lychee and the Hugo docs build in the CI images. It also ran Bitnami builds of the MinIO releases: minio gateway azure starts on the 26 March 2022 release, the 29 April release still lists azure under minio gateway, and from the 30 April release the command fails as an unknown subcommand.

Related issues

None.

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