doc: The data table <-> vector index table pattern

[asg017]

create table xyz(a, b, c);

create virtual table vss_xyz using vss0(
  a_embedding(1024), 
  b_embedding(1024),
  c_embedding(1024)
);

-- optional: doc why this could be nice
create table xyz_embeddings(
  a_embedding vector(1024), -- doc why vector(1024) or other options
  b_embedding vector(1024),
  c_embedding vector(1024),
);