Move benchmarks to xdsl-bench and add documentation

[EdmundGoodman]

Summary

Remove the benches/ directory, as it now duplicates the content of the xdsl-bench repository, which is the new benchmarking suite under active development and tracking performance regressions at https://xdsl.dev/xdsl-bench/.

Changes

• Update README to discuss benchmarking
• Remove benches/ directory
• Remove asv.conf.json

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.31%. Comparing base (b943df3) to head (7b2ab87).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3713      +/-   ##
==========================================
- Coverage   91.32%   91.31%   -0.01%     
==========================================
  Files         467      468       +1     
  Lines       58543    58546       +3     
  Branches     5652     5652              
==========================================
  Hits        53463    53463              
- Misses       3628     3631       +3     
  Partials     1452     1452              

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[superlopuh]

Excellent