Replace Vespa references with OpenSearch

[rohoswagger]

Summary

Vespa was removed in v4; the current-state docs still described it as the document index. This swaps every such prose reference to OpenSearch and replaces the Vespa Configuration env-var accordion in configuration.mdx with the real OPENSEARCH_* variables from backend/onyx/configs/app_configs.py (host/port, admin creds, TLS/mTLS, OPENSEARCH_AUTH_METHOD basic/iam, timeouts, analyzer, shards/replicas).

Intentionally left as-is:

• changelog.mdx and opensearch_document_index_migration.mdx — historical / migration content where "Vespa" is the correct word.
• vespa-dedicated=true:NoSchedule taint in terraform.mdx — still the literal taint key used by the eks terraform module.
• vespaengine/vespa:... in release_process.mdx — this is what https://cloud.onyx.app/api/versions actually returns today.
• LOG_VESPA_TIMING_INFORMATION — still the actual env var name; only its description was reworded.

The EKS teardown PVC name was updated to the opensearch subchart's default (onyx-opensearch-master-onyx-opensearch-master-0, from clusterName/nodeGroup in the Helm values).

Link to Devin session: https://app.devin.ai/sessions/3644b0cb6a6f453580498ccf04eb0121
Open in Devin Desktop: https://app.devin.ai/desktop/session/3644b0cb6a6f453580498ccf04eb0121?variant=devin
Requested by: @rohoswagger

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