docs: Include full policy for avatars and uploads buckets.
docs: Include full policy for avatars and uploads buckets.

The documentation included the full policy for the file uploads
bucket, but only one additional statement for the avatars bucket; the
reader needed to assemble the full policy themselves.

Switch to explicitly providing the full policy for both.

Fixes #23110.

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip