[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 #224, therefore this PR need to wait after it is merged.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

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