docs(python): Add documentation for beta gpu support

[beckernick]

This PR adds initial documentation for the beta Polars GPU engine. It includes:

• A general overview of the beta GPU engine in the user guide, including information on what's supported, how it works, testing, interoperability, installation, and similar
• Updated docs for the GPUEngine configuration object
• Minor updates to API docs where relevant

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.88%. Comparing base (4894e24) to head (ffc6262).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18762      +/-   ##
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+ Coverage   79.85%   79.88%   +0.03%     
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  Files        1517     1517              
  Lines      205530   205533       +3     
  Branches     2892     2892              
==========================================
+ Hits       164119   164191      +72     
+ Misses      40863    40794      -69     
  Partials      548      548              

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