docs: improve README with clearer raw vs optimized streaming options

[bhimrazy]

What does this PR do?

Improved the "Speed up model training" section in README.md to better explain the two main approaches for data streaming with LitData.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84%. Comparing base (21786d1) to head (b365731).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@         Coverage Diff         @@
##           main   #677   +/-   ##
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  Coverage    84%    84%           
===================================
  Files        52     52           
  Lines      7143   7143           
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+ Hits       5965   5967    +2     
+ Misses     1178   1176    -2     

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