Add WebEngage integration screenshots

[tashviks]

Summary

• Expand the WebEngage analytics integration guide with a customer-facing setup flow.
• Add screenshots showing how to reach WebEngage REST API credentials from Data Platform > Integrations > REST API.
• Add a WebEngage Project Credentials screenshot for License Code and API Key for Team Member.
• Add a Linkrunner WebEngage integration drawer screenshot showing where to paste License Code, API Key, and Data Center Region.
• Add a WebEngage Event Analytics screenshot showing how the inorganic_install event appears post setup.
• Document the credential field mapping, data center host mapping, server-side API key handling, and Sync with WebEngage behavior.

Validation

• Ran git diff --cached --check before commit.
• Ran git diff --check -- analytics-integrations/webengage.mdx images/webengage after final screenshot/doc edits.

Notes

• No code changes. Docs and screenshots only.