[3.13] Docs: Move inline JavaScript to own file to reduce duplication (GH-119541)

[hugovk]

(cherry picked from commit eea26c4)

This also adds:

       {
           slug: "dev (3.14)",
           urls: {
               documentation: "https://docs.python.org/3.14/",
           }
       },

Which was added to main in #119539 and not for backporting in whole.

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📚 Documentation preview 📚: https://cpython-previews--119630.org.readthedocs.build/