Docs: DOC-3530 - Update registry login username to tiny on AI on-premises page (#4199)

Replace the <registry-username> placeholder with the actual username tiny
in the Docker and Podman login commands, and update the surrounding
guidance accordingly. All accounts authenticate as tiny against
registry.containers.tiny.cloud, consistent with the other on-premises
installation pages.

Author: kemister85

modules/ROOT/pages/tinymceai-on-premises-getting-started.adoc

@@ -48,18 +48,18 @@ For Docker:
 
 [source,bash]
 ----
-docker login -u '<registry-username>' https://registry.containers.tiny.cloud
+docker login -u tiny https://registry.containers.tiny.cloud
 # Docker prompts for the password; this avoids leaking it in shell history.
 ----
 
 For Podman:
 
 [source,bash]
 ----
-podman login -u '<registry-username>' registry.containers.tiny.cloud
+podman login -u tiny registry.containers.tiny.cloud
 ----
 
-Replace `<registry-username>` with the username supplied by the Tiny account representative. If credentials have not been received, contact `support@tiny.cloud`.
+All accounts authenticate with the username `tiny`. When prompted, enter the access token supplied by the Tiny account representative. If credentials have not been received, contact `support@tiny.cloud`.
 
 === Pull the AI service image