docs: document graph backend configuration

[erenbati]

Fixes #278

Summary

• Document the graph backends registered by guacgql and how to select them with gql-backend.
• Document the keyvalue backend options, including kv-store, Redis, and TiKV configuration.
• Clarify that the default memmap store is in-memory and does not persist graph data across guacgql restarts.
• Document the ArangoDB, Neo4j, and Neptune backend-specific configuration options and their relevant defaults.
• Update the configuration defaults table to distinguish demo guac.yaml values from binary defaults.

Validation

• npx --yes prettier --check --prose-wrap always ./**/*.md
• Passed in GitHub Actions on the fork.

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