change extension index link to the new dedicated repo instead of wiki

            with gr.TabItem("Available"):

                with gr.Row():

                    refresh_available_extensions_button = gr.Button(value="Load from:", variant="primary")

                    available_extensions_index = gr.Text(value="https://raw.githubusercontent.com/wiki/AUTOMATIC1111/stable-diffusion-webui/Extensions-index.md", label="Extension index URL").style(container=False)

                    available_extensions_index = gr.Text(value="https://raw.githubusercontent.com/AUTOMATIC1111/stable-diffusion-webui-extensions/master/index.json", label="Extension index URL").style(container=False)

                    extension_to_install = gr.Text(elem_id="extension_to_install", visible=False)

                    install_extension_button = gr.Button(elem_id="install_extension_button", visible=False)