[p5.js 2.0 Bug Report]: Add docs about the 2.0 reference-generation process

[nbogie]

Most appropriate sub-area of p5.js?

• Accessibility
• Color
• Core/Environment/Rendering
• Data
• DOM
• Events
• Image
• IO
• Math
• Typography
• Utilities
• WebGL
• Build process
• Unit testing
• Internationalization
• Friendly errors
• Other (specify if possible)

p5.js version

v2.x

Web browser and version

n/a

Operating system

n/a

Steps to reproduce this

I think there's a use for documentation around the build process for the 2.0 reference (and type-declarations).

Splitting this new issue out from #8454 to deal only with the build process.