docs: migrate document builder to mkdocs

[geniuspatrick]

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• Your code builds clean without any errors or warnings
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Motivation

We are going to migrate our document builder for sphinx to mkdocs!

MkDocs is a fast, simple and downright gorgeous static site generator that's geared towards building project documentation. Documentation source files are written in Markdown, and configured with a single YAML configuration file.

Life is short, use mkdocs to write documents.

The structure of our documents is planned as follows:

• Home
• Installation
• Model Zoo
• Tutorials: # Learning Oriented
  • Quick Start
  • Configuration
  • etc.
• How-To Guides: # Problem Oriented
  • Write A New Model
  • CLS/DET/SEG
  • etc.
• Reference: The API Reference
• Notes:
  • Change Log
  • Contributing
  • etc.

Test Plan

I added an action workflow to check if the document build was successful.

Related Issues and PRs

(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)