Update hiredis usage instructions in documentation

[gabang2]

The documentation for enabling the hiredis parser is outdated. It currently instructs users to set the PARSER_CLASS option, which is no longer the recommended or necessary approach.

redis-py automatically detects and uses the high-performance hiredis parser if the package is installed in the environment. No additional configuration is required from the user.

This PR updates the "Pluggable parsers" section in README.rst to align with the current behavior by:

• Removing the obsolete PARSER_CLASS configuration example.
• Clarifying that hiredis is used automatically by default when installed.
• Providing the simple pip install hiredis command as the only required step.

This change makes the documentation more accurate and user-friendly, ensuring that users can easily leverage the performance benefits of hiredis without confusion.

Depends on :

• Outdated documentation about hiredis #750 (comment)

[WisdomPill]

I will take some time to verify if we need to remove the default parser, cause if redis already does the parsing we might need to not redo it

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.5%. Comparing base (9423cee) to head (2c0dad6).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #791   +/-   ##
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  Coverage    38.5%   38.5%           
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  Files          44      44           
  Lines        3320    3320           
  Branches      245     245           
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  Hits         1276    1276           
  Misses       1799    1799           
  Partials      245     245           

Flag	Coverage Δ
mypy	38.5% <ø> (ø)

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