docs: MkDocs use of snippets for README, CONTRIBUTING, LICENSE

[fyliu]

Overview

We need to document how we set up the project so that contents exist in a single location but satisfies both Github and MkDocs requirements that they be in different places. It's through MkDocs snippets.

Action Items

• add documentation for snippets and how we use it in the project documentation

Resources/Instructions

• we keep the content files in the project root and use snippets inside the docs/ directory so to keep MkDocs happy
• The LICENSE file exists in upper and lowercase intentionally. There's places that need both.