[510] Document listener configuration

[NanaKhadija1980j]

Summary

• Documented listener configuration options.
• Added defaults and recommended values.
• Added practical configuration examples.
• Documented required and optional settings.

Closes #510

Test plan

• Cross-checked configuration options against listener/src/config.ts, archive-config.ts, logger.ts, and index.ts.
• Verified documented defaults against loadConfig() / loadArchiveConfig().
• Verified configuration examples match .env / JSON formats used by the project.
• Verified internal documentation links.

Validation

• Options and defaults traced to source (config.ts, archive-config.ts, logger.ts, Discord overrides in index.ts).
• Noted implementation gap: API_KEYS is documented in .env.example / validateApiKeys, but loadConfig() currently references undeclared rawApiKeys.
• Noted listener/.env.staging uses PORT / RPC_URL, which loadConfig() does not read.
• No markdownlint tooling is configured in this repository; validation was manual path/link/default checks.
• No application logic changes.

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