[v1.38] Drop vector index

[g-despot]

Summary

Document the new delete_vector_index API for dropping a named vector index from a collection. This is a destructive, irreversible operation — the index is removed from disk but vector data is preserved.

• How-to section in manage-collections/vector-config.mdx with Python code example
• Reference section in config-refs/indexing/vector-index.mdx with REST endpoint
• Verified against Weaviate 1.37.0-rc.0

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

[orca-security-eu]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Secrets	0   0   0   0	View in Orca