Clarify Qubit state descriptions and aliases

[sotashimozono]

Key changes:

• Fixed incorrect eigenvalues for $\sigma_x$ and $\sigma_y$ eigenvectors.
• Added a code example for state to match the format of the "Operator" section.
• Clarified that "0" and "1" states are eigenvectors of $\sigma_z$ for consistency.

Description

While reviewing the documentation, I noticed that some eigenvalues for the "Qubit" states were inverted, so I have corrected them.
To improve consistency with the "Operator" section and make the guide more practical, I have added a brief code example showing how to prepare states using the state function. Additionally, I explicitly mentioned that the "0" and "1" states are eigenvectors of $\sigma_z$ to align with the descriptions of the other bases.Please feel free to adjust or remove any parts you find redundant.

How Has This Been Tested?

I checked each alias itself works consistently, and documents had mistakes.
So, I only fixed markdown file.

Checklist:

• My code follows the style guidelines of this project. Please run using JuliaFormatter; format(".") in the base directory of the repository (~/.julia/dev/ITensorMPS) to format your code according to our style guidelines.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that verify the behavior of the changes I made.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.

[sotashimozono]

Just a trivial Documentation Issue.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.31%. Comparing base (4b29909) to head (cc51cee).

Additional details and impacted files

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##             main     #174   +/-   ##
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  Coverage   90.31%   90.31%           
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  Files          54       54           
  Lines        3605     3605           
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  Hits         3256     3256           
  Misses        349      349           

Flag	Coverage Δ
docs	18.00% <ø> (ø)

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[emstoudenmire]

Thanks for catching the issue and for the suggested changes. I think the changes mostly look good, except for one which I commented on above.

[sotashimozono]

thanks, let me think about the changes.

[sotashimozono]

Thanks for the feedback! I've added the general examples for siteind, state, and op at the beginning of the file so we don't have to repeat them everywhere.

For the individual sections, what do you think about the existing code snippets? I'm happy to delete them to avoid duplication, though I'd prefer to keep the scope of this PR manageable unless you feel strongly about a full cleanup. Let me know your thoughts.