feat(ExternalStorage) Allow the use of 'prefix' in S3 buckets

[mbartsch]

☑️ Resolves

• Companion to nextcloud/server#61585

🖼️ Screenshots

Before — no prefix field mentioned in the S3 external storage documentation.

After — new section between the Bucket and Access key fields:

In the Bucket field enter your S3 bucket name. [...]

The optional Object key prefix field scopes the mount to a sub-path within
the bucket rather than using the entire bucket. For example, setting the prefix
to nextcloud/ will store all files under the nextcloud/ key namespace
inside the bucket, leaving the rest of the bucket untouched. This is useful
when a bucket is shared between multiple services or Nextcloud instances.

.. note::
The prefix is automatically normalized: leading slashes are stripped and a
trailing slash is always added. myapp, myapp/, and /myapp/ are
all equivalent.

.. tip::
The $user placeholder is supported in the prefix field and is substituted
with the Nextcloud username at mount time. For example, setting the prefix to
nextcloud/$user/ will store each user's files under their own namespace
(e.g. nextcloud/alice/ for user alice). This is particularly useful
for global mounts shared across all users.

In the Access key field enter your S3 access key ID.

✅ Checklist

• I have built the documentation locally and reviewed the output
• Screenshots are included for visual changes
• I have not moved or renamed pages (or added a redirect if I did)
• I have run codespell or similar and addressed any spelling issues

Assisted-by: CloudeCode:claude-sonnet-4-6