[doc] Add more examples and detailed docstring for functions in different modules

[Routhleck]

• [doc] Detailed docstring for functions in _base.py #20
• [doc] Detailed docstring for functions in _celsius.py #21
• [doc] Detailed docstring for functions in autograd module #22
• [doc] Detailed docstring for functions in fft module #23
• [doc] Detailed docstring for functions in linalg module #24
• [doc] Detailed docstring for functions in math module #25