docs: add Drawings documentation and site listings

[antonreshetov]

Summary

Documents the Drawings (Excalidraw) space and lists it across the docs site.

• New page documentation/drawings/ — overview with When to use, creating, managing (rename, duplicate, export, delete), list search with arrow navigation, the canvas and fit-to-content, embedding in notes, and storage. Tagged >=5.6.
• Navigation & SEO — added Drawings to the docs sidebar and to the site/page descriptions.
• Landing — added a Drawings feature card (and rebalanced the features grid) and mentioned drawings in the hero and meta description.
• Marketing pages — added Drawings to the space inventory on the compare index, the per-competitor compare pages, and the sponsor page.
• README — added a Drawings feature section and updated the summary lines.
• Added drawings.png and notes-drawings.png screenshots.