content: add Herman Tcheneghon Motcheyo as a PhD student

[chengjiahao1234]

Type of Change

• Content Update (team)

Summary

Add Herman Tcheneghon Motcheyo to the PhD Students section.

Changes Made

• Append Herman to _data/phd_students.yml, preserving existing member order.
• Add his portrait: images/teampic/herman_tcheneghon_motcheyo.jpg.
• Include his website, GitHub, LinkedIn, and Google Scholar links.
• Display his Concordia email; retain his personal email as a non-displayed YAML comment.

Testing Checklist

• Desktop (1920x1080)
• Tablet (768x1024)
• Mobile (375x667)
• Light theme
• Dark theme
• Portrait loads correctly; no layout overflow
• No browser console errors
• Ran bundle exec jekyll serve successfully
• Verified website, GitHub, and Google Scholar
• Passed YAML checks and git diff --check

Screenshots

Screenshots captured locally for all tested viewports and themes.

Related Issues

None.

Additional Notes

• The supplied local portrait is a valid JPEG, not a PNG.
• LinkedIn identity was verified through its indexed profile and
  cross-links; direct automated access is blocked by LinkedIn.
• No unrelated content or styling changes.

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By submitting this PR, I confirm:

• I have tested these changes locally
• I have followed the code standards in DEVELOPMENT.md
• Documentation updates are not required
• I am ready for this to be reviewed and merged