docs: mark project as deprecated, link to official Solcast SDK

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

Author: mcaulifn

README.rst

@@ -2,6 +2,8 @@
 pysolcast
 =========
 
+⚠️ **DEPRECATED** — Solcast now maintains an official Python SDK. Please use `solcast-api-python-sdk <https://github.com/Solcast/solcast-api-python-sdk>`_ instead.
+
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