Remove margin from PageHeader

[SteveMicroNova]

What does this change intend to accomplish?

I noticed there was a thin black strip at the top of the screen on the About page, and found it was due to the PageHeader component having a 0.5 rem margin-top

This PR fixes that

Before:

After:

Checklist

• Have you tested your changes and ensured they work?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Does your submission pass linting & tests? You can test on localhost using ./scripts/test
• If this is a UI change, have you tested it across multiple browser platforms on both desktop and mobile?

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.81%. Comparing base (7499989) to head (9fd12ae).
Report is 64 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1027      +/-   ##
==========================================
- Coverage   50.67%   49.81%   -0.86%     
==========================================
  Files          40       41       +1     
  Lines        7154     7303     +149     
==========================================
+ Hits         3625     3638      +13     
- Misses       3529     3665     +136     

Flag	Coverage Δ
unittests	49.81% <ø> (-0.86%)	⬇️

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