document the gen2 additions

[OSSDiablo]

companion docs for the gen2 PRs. all of it is new pages and new sections, nothing existing is rewritten.

• label and paragraph, one page each, for Add Label and Paragraph rayfield-gen2#90
• divider, for #92
• a gradients section on the colour picker page, for #89
• a resizing section on the windows page for the corner grip and the two settings, for #86

screenshots are pulled from the recordings on those PRs and cropped, so they are the elements as they actually ship.

worth holding until the matching PRs merge, since none of this exists on dev yet. happy to split it per feature if you would rather review it that way.