DT-8885: Update provider registry documentation with detailed descriptions and usage guides

[snirzango]

Jira: https://datafyio.atlassian.net/browse/DT-8885

Changes

• Rewrote docs/index.md with comprehensive provider documentation including:
  • Clear provider overview explaining the relationship between datafyaws and the standard AWS provider
  • Prerequisites section (Datafy org account, API token, AWS credentials)
  • Expanded example usage with EBS volume, volume attachment, and EC2 instance resources
  • Documented Datafy-specific provider arguments (datafy_token, datafy_url) with environment variable alternatives
  • Detailed "Resources Supported by Datafy" section explaining behavior for each resource type
  • Important behavior notes covering managed volumes, volume replacement, snapshot restores, and Datafy tag filtering
  • Links to docs.datafy.io for further reading

Test Plan

• Verify the documentation renders correctly on the Terraform registry
• Confirm all links to external documentation (AWS provider docs, docs.datafy.io) are valid
• Review terminology and descriptions match the Datafy UI and platform behavior