[doc] Update documentation

[Routhleck]

New outline of documentation:

• Index
• Physical Unit
  • Quantity
    • mantissa * scale (unit)
    • Creating Quantity Instance
      • Scalar and Array Multiplication
      • Direct Quantity Creation
      • Unitless Quantity
      • Illegal Quantity Creation
      • Creating Functions
    • Converting to Different Units
    • Attributes of a Quantity
  • Math Operations with Quantity
    • Addition and Subtraction
    • Multiplication and Division
    • Power
    • Built-in Functions
    • Indexing, Slicing and Iterating
    • Operating on subsets
      .at property
  • Standard Units
    • 增加example
    • 组合unit由 standard unit组合而成
  • Constants
    • 去除小标题
    • 增加每个constants的介绍
• Unit-aware Math Functions
  • Array Creation
    • brainunit.math.array & brainunit.math.asarray
    • brainunit.math.arange
    • brainunit.math.array_spilt
    • brainunit.math.linespace
    • brainunit.math.logspace
    • brainunit.math.meshgrid
    • brainunit.math.vandermode
    • Can use with Quantity
      • brainunit.math.full
      • brainunit.math.empty
      • brainunit.math.ones
      • brainunit.math.zeros
      • brainunit.math.full_like
      • brainunit.math.empty_like
      • brainunit.math.ones_like
      • brainunit.math.zeros_like
      • brainunit.math.fill_diagonal
    • Can use with unit keyword
      • brainunit.math.eye
      • brainunit.math.identity
      • brainunit.math.tri
      • brainunit.math.diag
      • brainunit.math.tril
      • brainunit.math.triu
  • NumPy Functions for Quantity
    • Functions
      • Array Creation Functions
      • Functions that Changing Unit
      • Functions that Keeping Unit
      • Functions that Removing Unit
      • Functions that Accepting Unitless
      • Einstein Operations
      • Ohter Functions
    • Shape Manipualtion
    • Comparing Quantity
  • Elinstein Operations
  • Functions that Checking Units
• Advanced Guide
  • Combining and Defining Unit
    • Combining Example
    • Defining Units
      • Creating Basic Units
      • Creating Compound Units
      • Creating Scaled Units
  • Mechanism of Quantity
    • Introduction
    • Dimension
    • Quantity
    • Units
• API Documentation