feat: add the tab_style_body() method

[rich-iannone]

This PR adds a new value-driven table styling method, tab_style_body(). It allows users to apply styles to table cells based on their data values rather than their positions. The user guide has been updated with usage examples and explanations to help users with the new method.

[codecov]

Codecov Report

❌ Patch coverage is 98.73418% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.69%. Comparing base (6948573) to head (12c1e08).

Files with missing lines	Patch %	Lines
great_tables/_tab_create_modify.py	98.71%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #883   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          51       51           
  Lines        7746     7824   +78     
=======================================
+ Hits         7645     7722   +77     
- Misses        101      102    +1     

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