[docs] Update more detailed and clearer documentation

[Routhleck]

Physical Units

• index
• Quantity
  • Creating Quantity Instances
  • Converting to Different Units
  • Attributes of a Quantity
  • Arithmetic Functions
    • Addition and Subtraction
    • Multiplication and Division
    • Power
  • Built-in Functions
  • Indexing, Slicing and Iterating
• Standard Units
  • Prefix
• Constants
• Mechanisms of Quantity / Dimension / Unit
• Quantity Conversion for Other Utilities
  • Dimensionless Quantities
  • Plotting Quantities
  • Converting to Plain Python Scalars
• Combining, Defining, and Displaying Units
  • Basic example
  • Defining units
  • Displaying in JIT / grad / ... transformations

Mathematical Functions

• index
• NumPy Functions
  • Shape Manipulation
    • Changing the shape of a Quantity
    • Stacking together different Quantities
    • Splitting one Quantity into several smaller ones
  • Comparing Quantities
• Customize Functions that Accept Quantities

[chaoming0625]

great work.