Update webhook payload examples with complete structures

[devin-ai-integration]

Description

Updated webhook payload documentation to include complete, comprehensive payload examples instead of incomplete { ... } placeholders that weren't helpful for developers integrating with Statsig webhooks.

Key changes:

• Replaced "user": { ... } with complete user objects showing userID, email, country, locale, appVersion, and custom fields
• Replaced "statsigMetadata": { ... } with complete metadata showing sdkType, sdkVersion, stableID, sessionID, and environment
• Added missing userID and environment fields to config change webhook examples
• Fixed a missing comma in the batch example JSON
• Updated all 4 webhook types: custom events, gate exposures, config exposures, and experiment exposures

Best practice checklist

• I've considered the best practices on where to put your doc and what to put in your doc
• I've deleted and redirected old pages to this one, if any
• I've updated links affected by this change, if any
• I've updated screenshots affected by this change, if any

Human Review Checklist

Please verify:

• Field accuracy: Do the payload structures match actual webhook payloads sent by Statsig?
• Example values: Are the example values (userIDs, emails, etc.) appropriate and not misleading?
• Missing fields: Are there any important fields in real payloads that should be included in these examples?
• Field presence: Do all the fields I included (like stableID, sessionID, environment) actually appear in webhook payloads consistently?

Questions?

Reach out to Brock or Tore on Slack!

---

Link to Devin run: https://app.devin.ai/sessions/a8df8caf96c44f4ca9de463ee3224695
Requested by: @lfoster-statsig via Slack #bot-devin-ai channel

Note: The payload structures were researched from existing codebase examples, but should be verified against actual webhook data to ensure accuracy.

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring

[jasonwzm]

🚀 Deployed on https://68d48cce85a19a5443a477d8--cozy-fox-0defec.netlify.app