Add Backspread Option Strategy

[LouisSzeto]

Description

Add documentations of OptionStrategies.CallBackspread, OptionStrategies.PutBackspread, OptionStrategies.ShortCallBackspread, OptionStrategies.ShortPutBackspread in Writing Algorithms / Trading and Orders / Option Strategies

Related Issue

Motivation and Context

newly merged Lean function

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] Refactor (non-breaking change which improves implementation)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Non-functional change (xml comments/documentation/etc)

Checklist:

• [X] My code follows the code style of this project.
• [X] I have read the CONTRIBUTING document.
• [X] My branch follows the naming convention bug-<issue#>-<description> or feature-<issue#>-<description>