Add Clear Linux OS deprecation notice

This text is copied from the official announcement at https://community.clearlinux.org/t/all-good-things-come-to-an-end-shutting-down-clear-linux-os/10716 (I only added the date of the announcement to clarify when "immediately" means since the image description is a more static context than the forum post).

clearlinux/README-short.txt

@@ -1 +1 @@
-Official docker build of Clear Linux OS for Intel Architecture
+DEPRECATED; Official docker build of Clear Linux OS for Intel Architecture

clearlinux/deprecated.md

@@ -0,0 +1,3 @@
+https://community.clearlinux.org/t/all-good-things-come-to-an-end-shutting-down-clear-linux-os/10716
+
+> Effective immediately [2025-07-18], Intel will no longer provide security patches, updates, or maintenance for Clear Linux OS, and the Clear Linux OS GitHub repository will be archived in read-only mode. So, if you’re currently using Clear Linux OS, we strongly recommend planning your migration to another actively maintained Linux distribution as soon as possible to ensure ongoing security and stability.