Slate v2 Editable Docs Truth Pass

Goal

Align the public docs with the actual rich-text editing surface: EditableBlocks / EditableTextBlocks for structured rendering, Editable for the lower-level DOM snapshot seam.

Current Batch

1. fix Editable reference docs so they describe the current low-level surface
2. retarget stale rendering/walkthrough examples to EditableBlocks
3. remove overclaims for unsupported legacy props like renderPlaceholder and renderText
4. sync RC truth docs if the public guidance changes materially

Result

• rewrote the Editable reference doc so it describes the current low-level DOM snapshot seam and the structured EditableBlocks / EditableTextBlocks surface separately
• updated the main install, rendering, TypeScript, event-handling, formatting, commands, persistence, and collaboration docs to use EditableBlocks for the structured rich-text surface
• recovered renderElement.attributes on EditableBlocks, which makes the current example shape honest again for callers who want to own the element host directly
• updated the RC proof ledger to call out the recovered render-element host seam

Verification

• targeted readback on the touched docs
• yarn workspace slate-react run test -- --test-name-pattern "EditableBlocks exposes renderElement attributes so callers can own the element host|EditableBlocks exposes renderLeaf so callers can own the leaf host"
• yarn workspace slate-react run test
• yarn test:custom
• yarn lint:typescript