Add page in the custom extensions docs for adding buttons to the Selection Toolbox

[christian-byrne]

See https://github.com/Comfy-Org/ComfyUI_frontend/blob/f987cf9dbdc8eabf4c5e7eed65473d48c316783d/src/types/comfy.ts#L102-L107

This is part of the public extension API. The page can go next to the pages for toasts, bottom panels, sidebar tabs, and so on.