Docs: update uvloop how-to to avoid deprecated EventLoopPolicy

[veeceey]

Summary

• Replaces the deprecated uvloop.EventLoopPolicy() pattern in docs/how-to-guides/uvloop.rst with a minimal custom policy subclass that delegates to uvloop.new_event_loop()
• Adds a .. note:: block explaining the deprecation context (uvloop#637, Python 3.14 policy deprecation) and linking to #1164 for the planned non-policy-based replacement
• Follows the same deprecation-warning suppression pattern already used in multiple_loops_example.py and the fixtures reference examples

Closes #1346

Test plan

• Verify the RST renders correctly in the Sphinx docs build (tox -e docs or equivalent)
• Confirm the code example works with a recent uvloop (>= 0.21) by placing it in a conftest.py and running an async test

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.64%. Comparing base (cb129a6) to head (7dd8396).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1355   +/-   ##
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  Coverage   93.64%   93.64%           
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  Files           2        2           
  Lines         409      409           
  Branches       44       44           
=======================================
  Hits          383      383           
  Misses         20       20           
  Partials        6        6           

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