docs refine: kubernetes

[cutecutecat]

• Set [Build PostgreSQL image with pgvecto.rs] as optional
• Add recommend image name imageName: <your-postgresql-image-repo>:<pg-tag> to ghcr.io/tensorchord/cloudnative-pgvecto.rs:16-v0.2.0
• Add step to apply yaml: kubectl apply -f cnpg.yaml at [Create PostgreSQL cluster]