docs: add evals documentation, update commands reference and configuration guide

[notgitika]

Summary

• Add docs/evals.md — comprehensive guide covering evaluators, on-demand evals, online eval configs, rating scales, placeholders, sampling rate, builtin evaluators, standalone (ARN) mode, and common patterns (CI/CD quality gates)
• Update docs/commands.md — add command reference for add evaluator, add online-eval, run evals, evals history, pause/resume online-eval, logs evals
• Update docs/configuration.md — add evaluator and online eval config schema documentation, update project fields table and sample agentcore.json
• Update README.md — add Evaluations to capabilities, commands table, documentation links, and project structure description

Test plan

• Verify all doc links resolve correctly
• Verify CLI examples match actual command flags
• Verify schema examples match current Zod schemas

[tejaskash]

LGTM