Update "Icons > requesting-an-icon" documentation

[dancormier]

Our internal instructions don't match up with our documentation on Stacks. We should remedy this.

There's two steps to go about submitting a new Spot illustration (or an Icon in general) to the Stacks-Icons repo:

Reach out to a designer to create the spot/icon in svg format

Open a pull request at the [Stacks-Icons GitHub repo](https://github.com/StackExchange/Stacks-Icons) with the svg added to the appropriate folder
    /src/{Icons,Spots}/IconName.svg

edit We've added requirement checklists to the repo as PR templates (icons here, spots here). Use those as the official source of truth in regards it svg requirements.

That's it! A few things to note about the svg itself:

The svg does not need to be optimized. In fact, the more readable the better. All svgs are automatically optimized as part of the package build/delivery step

Icons/spots with "native" full-color are to be submitted with all color intact. Making the icons mono-color is [taken care of via css](https://github.com/StackExchange/Stacks/blob/develop/lib/css/base/icons.less#L10-L20), so you do not need to worry about making two different versions of the same icon/spot.

Icons need to be perfectly aligned to the pixel grid. Avoiding sub-pixels keeps the image sharp

If the spot/icon is intended to be used at different sizes, consider including a SpotSm/SpotLg/SpotXL version as well

If you intend to use the icon via css (fairly rare, done for high perf reasons), include that info in the PR, as that support needs to be added on an icon-by-icon basis