Add the How-To Guide for Multi-Scale Feature Extraction

[DexterJZ]

Add the EN&CN versions of the how-to guide for multi-scale feature extraction.

Thank you for your contribution to the MindCV repo. Before submitting this PR, please make sure:

• [x] You have read the Contributing Guidelines on pull requests
• [x] Your code builds clean without any errors or warnings
• [x] You are using approved terminology
• [ ] You have added unit tests

Motivation

(Write your motivation for proposed changes here.)

Test Plan

(How should this PR be tested? Do you require special setup to run the test or repro the fixed bug?)

Related Issues and PRs

702

(Is this PR part of a group of changes? Link the other relevant PRs and Issues here. Use https://help.github.com/en/articles/closing-issues-using-keywords for help on GitHub syntax)

[geniuspatrick]

1. 放在docs/en/how_to_guides文件夹下面
2. 添加一个中文翻译，没有就建一个空白文档，写上即将到来
3. 在mkdocs.yml里面第19行加上一条