Add manual page for premultiplied alpha blending

[Jordyfel]

The engine has options called premultiplied alpha in materials, shader blend modes and texture import settings, but the docs do not do a good job of explaining what they do and how to effectively utilize them. I was very confused when trying to use them at first so I'd like to add a documentation page in the manual.

I didn't manage to build the docs to preview the page for now, I will try again later.

[Jordyfel]

Thanks for the review, will do the images later.

[Jordyfel]

Converted the images and reworded a little at the end.

Resolved

[skyace65]

Hi Jordyfel, I've talked to some other maintainers and unfortunately this PR falls out of scope for what we want to have in the documentation.

Generally speaking we try to keep explanations for concepts that aren't Godot specific to a minimum. For example, we don't have a dedicated page breaking down how PBR materials work in a 3D scene, all major engines use them not just Godot. And it's the same with pre-multiplied alpha, that's not a Godot specific technology or way of doing things.

Putting that aside, a lot of the text in this PR is an over explanation for how this works. We generally strive to keep descriptions brief and accurate, if a page is filled with unnecessary information that's then more time a user has to spend finding what they need.

We appreciate the effort that you put into this PR, and I hope you'll consider contributing again in the future if you find something else you think could be improved. We have some guidelines if you want a more clear idea of what we're looking for: https://contributing.godotengine.org/en/latest/documentation/guidelines/index.html