Add a note about the deprecation of this extension (#337)

* Address the deprecation of this extension

* Added bold

---------

Co-authored-by: Manuel Kaufmann <[email protected]>

Author: agriyakhetarpal

README.rst

@@ -1,3 +1,15 @@
+.. warning::
+
+   **This extension has been deprecated**. Users are advised to enable the "Link previews" option in "Settings" > "Addons" > "Link previews"
+   in the Read the Docs admin dashboard for their project to get the same functionality as this extension.
+
+   For more information, please see:
+
+   1. https://docs.readthedocs.com/platform/stable/link-previews.html
+   2. https://about.readthedocs.com/blog/2024/07/addons-by-default/
+
+----
+
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 sphinx-hoverxref