[HIG-752] docs: fix subscription request field reference

[Dor-Skoler]

Summary

• Add missing required emails field to subscription request docs
• Correct tenant from required to optional
• Add optional secret and type fields to match the API schema (SubscriptionRequest)
• Replace bullet-point parameter list with a full field reference table and minimal body example

Test plan

• Verify field reference table renders correctly in GitBook
• Cross-check against SubscriptionRequest schema in swagger.yaml

🤖 Generated with Claude Code

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}