Improve README.

[vtjeng]

I updated the repository structure in this PR.

As part of my changes, I also added some improvements to the documentation:

• Applied prettier formatter to README. Among other things, this adds spaces after headers, aligns tables, and removes whitespace after lines.
• Remove broken link to documentation
• pip install -> python3 -m pip install
• Introduce MCCFR once, then use the acronym from then on.
• Spelling

[CLAassistant]

All committers have signed the CLA.