feat: add Terraform provider reference docs under Administration

[dangrondahl]

Summary

• Adds 7 new Mintlify pages under terraform-reference/ converted from the terraform-provider-kosli source docs (provider index, 3 resources, 3 data sources)
• Adds a Terraform Reference menu item in the Reference tab, positioned after Helm Reference
• Adds a Managing Custom Attestation Types page under Administration, following the same pattern as Managing Environments
• Updates administration/managing_environments/overview.md reference links to point to the new internal pages instead of the Terraform Registry

New pages

Reference → Terraform Reference

Page	Nav location
Provider overview	Reference → Terraform Reference → Provider
kosli_environment resource	Reference → Terraform Reference → Resources
kosli_logical_environment resource	Reference → Terraform Reference → Resources
kosli_custom_attestation_type resource	Reference → Terraform Reference → Resources
kosli_environment data source	Reference → Terraform Reference → Data Sources
kosli_logical_environment data source	Reference → Terraform Reference → Data Sources
kosli_custom_attestation_type data source	Reference → Terraform Reference → Data Sources

Administration → Managing Custom Attestation Types

A task-oriented how-to page covering:

• Creating custom attestation types via Terraform (schema + jq rules, jq-only, schema-only examples)
• Importing existing types into Terraform state
• Info callout linking to Getting started: Attestations for CLI usage and background
• Reference links to the Terraform reference pages

Content conversion

HashiCorp/tfplugindocs format was converted to Mintlify:

• page_title / subcategory frontmatter removed; title, description, icon added
• ~> Note: / ~> Important: callouts → <Warning> components
• -> Note: callouts → <Note> components
• H1 resource/data source headers removed (title comes from frontmatter)
• <!-- schema generated by tfplugindocs --> comments removed
• Internal cross-links use root-relative paths (/terraform-reference/...)

Test plan

• mint broken-links passes (no new broken links introduced)
• All 7 Terraform reference pages appear under Reference → Terraform Reference in local dev server
• Managing Custom Attestation Types appears under Administration in local dev server
• Callout components render correctly (no raw ~> or -> syntax)
• Cross-links from managing_environments/overview.md and managing_custom_attestation_types/overview.md resolve correctly