docs: migrate Replication section to v4 consolidated reference

Adds overview.md, clustering.md, and sharding.md for the Replication section.
Updates the v4 versioned sidebar and adds a link placeholder tracker.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

Author: Ethan-Arrowood

migration-context/link-placeholders/replication-link-placeholders.md

@@ -0,0 +1,11 @@
+# Link Placeholders for Replication
+
+## reference_versioned_docs/version-v4/replication/overview.md
+
+- Line ~84: `[Certificate Verification](TODO:reference_versioned_docs/version-v4/security/certificate-verification.md)`
+  - Context: Describing certificate revocation checking for replication connections — links to the cert verification config page for OCSP/CRL settings
+  - Target should be: `../security/certificate-verification.md`
+
+- Line ~183: `[Certificate Management](TODO:reference_versioned_docs/version-v4/security/certificate-management.md)`
+  - Context: "See Also" section footer link
+  - Target should be: `../security/certificate-management.md`