Improving documentation of importing resources

[mattdot]

This pull request includes several updates to the documentation and examples related to importing resources in Terraform. The most important changes include adding new import instructions for various resources, updating example scripts, and enhancing the documentation templates.

Documentation Updates:

• docs/guides/importing.md: Added a comprehensive guide on importing existing Power Platform resources into Terraform configurations. This includes sections on using terraform import, import blocks, and generating configurations.
• docs/resources/billing_policy.md: Added import instructions for billing policies.
• docs/resources/data_loss_prevention_policy.md: Added import instructions for DLP policies.
• docs/resources/environment.md: Added import instructions for environment resources.

Example Scripts:

• examples/resources/powerplatform_billing_policy/import.sh: Added an example script for importing billing policies.
• examples/resources/powerplatform_data_loss_prevention_policy/import.sh: Added an example script for importing DLP policies.
• examples/resources/powerplatform_environment/import.sh: Added an example script for importing environment resources.

Template Enhancements:

• templates/guides/importing.md.tmpl: Created a template for the importing guide to standardize documentation.
• templates/resources/environment.md.tmpl: Improved the template by adding trimmed descriptions and structured sections for examples and import instructions. [1] [2]

Codebase Updates:

• internal/powerplatform/services/environment/models_environment.go: Added "Default" to the list of environment types.