DX-3008: document vector index commands in upstash-redis-js

[CahidArda]

Adds data-structures/vector-indexes.md to upstash-redis-js. It covers the Redis 1.18 VECTOR.* commands through the redis.vector namespace:

• create or attach to an index;
• add vectors as number[], Float32Array, or { base64 };
• query, get, count, info, delete and drop;
• bulk loading through a pipeline;
• a short "Common Mistakes" list.

The page explicitly separates these Redis vector indexes from the standalone Upstash Vector database and says when to prefer @upstash/vector, so agents don't mix up the two products. The SKILL.md file index and description are updated the same way. The combined skills/upstash/ skill is regenerated with npm run build, and npm run check passes.

Merge after upstash/redis-js#1446 is released.

Linear: DX-3008

[linear-code]

DX-3008