docs: Sync configuration reference with current allocator schema

[7174Andy]

Summary

Updates docs/configuration.md to accurately reflect the current allocator configuration schema defined in structured_config.py, and removes advanced override documentation not needed by end users hosting tutorials.

Changes

Updated

• machine.ami_id default: ami-067cc81f948e50e06 (Ubuntu 20.04) → ami-00c257e12d6828491 (Ubuntu 24.04 + Docker + Nvidia)
• machine.repository default: documented as a URL → corrected to None (Optional[str])
• db.table_name default: vms → vm_table (matches schema)
• db.password / app.admin_password defaults: now show actual schema defaults instead of placeholder values
• dns.terraform_managed default: false → true (matches schema)
• AMI finder command updated from Ubuntu 20.04 to Ubuntu 24.04
• Table column alignment fixed for consistency

Added

• machine.extension field — file extension for software data files (default: slp)
• ssl.certificate_arn field — AWS ACM certificate ARN support
• ssl provider: acm — AWS Certificate Manager as fourth SSL provider option
• SSL Validation Rules subsection — documents cross-field dependency checks
• Allocator Deployment Options (allocator) section — documents image_tag for Terraform
• Monitoring Options (monitoring) section — full documentation for thresholds, budget, and CloudTrail config
• Configuration examples for each SSL provider

Removed

• Overriding Configuration section (5 override methods) — not needed by end users
• Configuration for Different Environments section (dev/staging/prod examples) — not needed by end users
• ssl.staging field — no longer exists in schema (replaced by provider: "none")
• Staging vs Production Mode section — replaced with provider-based documentation
• Allocator Options (client host/port) — was incorrectly placed in allocator config reference
• DNS fields app_name, pattern, custom_subdomain, create_zone — no longer in schema

Test plan

• Verify docs/configuration.md renders correctly with MkDocs (/docs-serve)
• Cross-check all defaults against packages/allocator/src/lablink_allocator_service/conf/structured_config.py
• Confirm no broken internal doc links

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