DOC: Add beginner-friendly introduction to Spatial Graph

[snehaseveriya23]

This PR adds a short introductory explanation to the Graph user guide to define what a Spatial Graph is and clarify its relationship to spatial weights.

Fixes #861

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.5%. Comparing base (7304239) to head (49bb965).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #862   +/-   ##
=====================================
  Coverage   85.5%   85.5%           
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  Files        151     151           
  Lines      16092   16092           
=====================================
  Hits       13758   13758           
  Misses      2334    2334           

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