Simplify documentation

[scarroll32]

Move to a easier documentation solution, Docusaurus has constant security updates and is flooding the issues / PRs.

• Review and simplify the docs with better examples
• Move back to a GitHub README only
• Redirect https://activerecord-hackery.github.io/ransack/ to the README

Discussed in #1360

^{Originally posted by RailsCod3rFuture September 9, 2022}
Docusaurus is showing up as a false positive in AWS. Is it mandatory to store it within the gem build folder in docs/?