[Docs] "Manipulating States and Operators" and "Tensor Products and Partial Traces"

[ytdHuang]

Summary of this PR:

• finish documentation section: Manipulating States and Operators
• finish documentation section: Tensor Products and Partial Traces
• move documentation section: The Importance of Type-Stability from Users Guide to Getting Started.
• add extended methods for expect and variance: allowing users to input a list (vector) of states
• add extended methods for tensor and kron: allowing users to input single QuantumObject or a Vector{QuantumObject} (but give a warning message regarding to type-instabilities) This part is also done in Fix ptrace and add extended methods for tensor and kron #224, therefore this PR need to wait after it is merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.85%. Comparing base (3f2eaab) to head (c619379).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #223      +/-   ##
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- Coverage   94.67%   93.85%   -0.83%     
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  Files          30       30              
  Lines        2124     2131       +7     
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- Hits         2011     2000      -11     
- Misses        113      131      +18     

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[albertomercurio]

I would prefer to put the hat on all the operators. Sorry to haven’t precise this before. If you don’t have time, don’t worry, I will change it later. You can merge it for me.