[wiki] Add the most basic example to Developing extensions page

[QuietNoise]

As per title. I suggest to add a very basic "hello world" example to the Developing extensions wiki page.
Currently the wiki page is referring to Developing custom scripts page to see how to develop custom scripts but sample on that page won't work when pasted as extension.
It won't work mainly because the sample script's show function returns is_img2img which won't work in extension.

    def show(self, is_img2img):

        return is_img2img

it took me some unecessarily wasted time to figure out that I need change it to

    def show(self, is_img2img):
        return scripts.AlwaysVisible

I'm sure every beginner to extensions development would appreciate if this was mentioned anywhere on the Developing extensions wiki page.

[ClashSAN]

sure, would you be able to provide an example?

[thr3a]

This repository is written in Japanese, but it may be helpful for you to create custom extension features.
https://github.com/aka7774/sd_extension_template

[ProGamerGov]

Is there an example for creating a custom page that performs a transform on an image without any rendering required?