Add note about contributing to other RTD projects (#12191)

Addresses #5676 

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- User's documentation (`docs`):
https://docs--12191.org.readthedocs.build/12191/

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- Developer's documentation (`dev`):
https://dev--12191.org.readthedocs.build/12191/

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Signed-off-by: Pavithra Eswaramoorthy <[email protected]>

Author: pavithraes

docs/dev/contribute.rst

@@ -32,6 +32,14 @@ starting with contributions, we suggest focusing on the following areas:
 .. contents::
     :local:
 
+.. tip::
+    You can also contribute to the `Read the Docs Sphinx Theme`_,
+    or Sphinx extensions in the `Read the Docs GitHub organization`_ like `sphinx-notfound-page`_.
+
+.. _Read the Docs Sphinx Theme: https://sphinx-rtd-theme.readthedocs.io/en/stable/
+.. _Read the Docs GitHub organization: https://github.com/readthedocs
+.. _sphinx-notfound-page: https://github.com/readthedocs/sphinx-notfound-page
+
 Contributing to development
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~