Switch to the MyST documentation system

MyST allows you to write Sphinx documentation entirely in markdown.

Here is a breakdown of the files that need to be converted:

The RST files under {{cookiecutter.project_name}}:

• The README
• The Contributor Guide
• The Code of Conduct. Use the original Markdown version provided by the Contributor Covenant.
• The licenses. Should we use the original plain text versions?

The RST files under {{cookiecutter.project_name}}/docs:

• The wrapper files: codeofconduct.rst, contributing.rst, license.rst
• The API reference: reference.rst
• The command-line reference: usage.rst
• The index: index.rst

Converting the Cookiecutter’s own documentation is tracked in a separate issue, see #1080.

[cauliyang]

Yep, that make sense! I can continue to convert other files under the doc to markdown. If you have any comments please free feel to tell me. It is my pleasure to contribute this project 😎

[cjolowicz]

Updated the issue description with a breakdown of the files that need to converted.