Document reanalyze

[fhammerschmidt]

• https://github.com/rescript-association/reanalyze
• https://github.com/rescript-lang/rescript-vscode#-code-analyzer
• https://dev.to/zth/getting-rid-of-your-dead-code-in-rescript-3mba

[fhammerschmidt]

Done in #1014