Docs: Add additional description

[nmerget]

• All icons should be monochromatic (check on the right in Figma's side panel to see how many Selection Colors there are).
• All icons should have the same "height." Otherwise, they will be "normalized" to the same height, which can lead to undesirable aesthetics, as visually smaller icons can suddenly appear relatively larger.
• To prevent this, place all icons in a uniformly sized frame, visually vertically centered and in the desired relative size. (Different widths are not a problem)
• The exported SVG should contain only a single path, as our font creation library ignores all additional paths. To prevent this, perform a "Union" of the individual paths.
• The exported frame should be named using kebab-case (lowercase letters with hyphens), without spaces, umlauts, or special characters. To avoid "blurry" icons, the paths should "snap" to the grid of the desired size.