[DOCS] Fix QN Index docs regarding dir keyword argument

[mtfishman]

See https://itensor.discourse.group/t/index-direction/977.

@emstoudenmire the controversial thing I did here was not show the dir keyword argument at all, I can't think of a reason why an end user should be using that rather than creating a new Index and calling dag on it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.44%. Comparing base (3fa632c) to head (8c76e20).
Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   79.44%   79.44%           
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  Files          64       64           
  Lines        5083     5083           
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  Hits         4038     4038           
  Misses       1045     1045           

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[emstoudenmire]

Yes, it's a good idea to just have users wrap the constructor in dag or call dag afterward rather than use the keyword argument. So I approve this PR.