DOC: expand CRS acronym in getting started guide

[mackenziereading19]

Part of #1507

Expands the first use of CRS in the Getting Started documentation to “Coordinate Reference Systems (CRS)” so the section is clearer to newcomers.

This is a documentation-only change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.84%. Comparing base (e81e5e5) to head (66fa2fc).
⚠️ Report is 53 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1640      +/-   ##
==========================================
- Coverage   95.85%   95.84%   -0.01%     
==========================================
  Files          20       20              
  Lines        1880     1879       -1     
==========================================
- Hits         1802     1801       -1     
  Misses         78       78              

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[djhoese]

If you're interested in helping with #1507, I'd love some extra eyes on my PR #1638.

[snowman2]

Thanks @mackenziereading19 👍