feat: Improve Documentation and Input Validation (#74)

* Initial plan for issue

* Implement comprehensive input validation and documentation improvements

- Add input validation for recovery_window_in_days (0 or 7-30 days)
- Add validation for automatically_after_days (1-365 days)
- Add validation for version_stages (AWSCURRENT/AWSPENDING only)
- Add validation for secret names (AWS naming requirements)
- Add validation for KMS key IDs (proper ARN/alias/key format)
- Add validation for tags (proper character restrictions)
- Add validation for rotate_secrets requiring rotation_lambda_arn
- Change variable types from any to proper structured object types
- Enhanced all variable descriptions with examples and constraints
- Add comprehensive type definitions for secrets and rotate_secrets objects
- Enhanced README with advanced usage examples (KMS encryption, policies, cross-region replication)
- Add comprehensive variable reference section
- Add detailed troubleshooting section with common issues and solutions
- Enhanced output descriptions with usage guidance
- Add input validation reference section

Co-authored-by: lgallard <6194359+lgallard@users.noreply.github.com>

* Revert variable types to 'any' while keeping loose validation for flexibility

Co-authored-by: lgallard <6194359+lgallard@users.noreply.github.com>

* fix: correct regex pattern for secret name validation to reject backslashes

Co-authored-by: lgallard <6194359+lgallard@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lgallard <6194359+lgallard@users.noreply.github.com>

Author: Copilot