Enhance Documentation with Subcategories for Resources and Data Sources

[publi0]

This PR addresses the documentation improvement requested in issue #1233. It introduces subcategories for all resources and data sources in the DigitalOcean Terraform provider documentation, significantly improving readability and navigation on the Terraform Registry page.

Key changes:

• Organized resources and data sources into logical subcategories
• Followed the structure of the DigitalOcean control panel menu for categorization
• Improved findability of related resources and data sources
• Enhanced overall documentation structure for better user experience

This update will make it easier for users to locate specific resources and data sources, understand their relationships, and navigate the provider documentation more efficiently.

Closes #1233

[andrewsomething]

🙏 Thank you! I think this will be a great improvement.

I did suggest one small change in-line to remain constant with the control panel's information hierarchy.

[andrewsomething]

👍 Thanks again for this great contribution!