GH-4281: Refresh Bedrock documentation links

[gaoxiaolei-s59]

Summary

• replace outdated Bedrock README links with the current Bedrock overview and Converse API references
• keep the embedding links that are still valid and collapse the old provider-specific chat links into the unified Converse page
• fix a stale internal Bedrock Converse xref to the current Bedrock reference page

Closes #4281

Verification

• ./mvnw -pl spring-ai-docs -DskipTests package