Update sparse vector documentation

[philippemnoel]

This is no longer correct, pgvector also supports sparse vectors now :)

[JinweiOS]

I can try to solve this problem. Is this column deleted directly here? or are there still some other requirements.

[gaocegege]

I think we could delete the column

[JinweiOS]

pgvector.rs-doc pr is #91, pgvector.rs pr is #595 and I saw a 404 link in comparison-pgvector.md L93.