Update docs reflecting v3 changes

[reck753]

Ready to merge when v3 release is published (not rc).

Here is what changed:

• updated getting started with new required package versions
• include how to setup rescript.json on Windows in getting started
• remove trailing units
• stop using Belt module in examples and assume RescriptCore is globally open
• use non-deprecated React hooks in examples (useCallback5 -> useCallback)
• stop using makeVariables in examples and remove it from docs

[zth]

Fantastic, thanks! Merging and releasing 3.0.0 soon.