[docs] Added certificate template screenshot for docs

[stktyagi]

Checklist

• I have read the OpenWISP Contributing Guidelines and OpenWISP Anti AI Spam Policy.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Description of Changes

Added updated image for certificate generator documentation.

[coderabbitai]

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[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (1 file)

• docs/1.4/certificate-templates/certificate-template.png

Previous Review Summary (commit 93546e6)

Current summary above is authoritative. Previous snapshots are kept for context only.

Previous review (commit 93546e6)

Status: No Issues Found | Recommendation: Merge

Files Reviewed (1 file)

• docs/1.4/certificate-templates/certificate-template.png

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_{Reviewed by balanced · Input: 32.8K · Output: 3.3K · Cached: 128.5K}