Slate v2 Migration/Performance Docs Truth Pass

Goal

Remove the last obvious stale docs that still teach plugin-stack, editor.exec, and renderChunk era ideas instead of the current recovered surface.

Result

• rewrote 09-performance.md around the current Slate / EditableBlocks / EditableTextBlocks surface
• removed stale renderChunk and chunking claims
• fixed the last install-guide provider note that still talked about the old value prop

Verification

• targeted grep/readback on the touched docs