Update p5.Element references

[nickmcintyre]

Preparing references in src/core/p5.Element.js for the new website.

PR Checklist

• npm run lint passes
• [Inline documentation] is included / updated

@Qianqianye @limzykenneth @davepagurek

[Qianqianye]

Looks good, thanks @nickmcintyre!
Just to clarify, in the new website, will the p5.Element methods (like parent (), toggleCalss() ) stay inside the p5.Element reference page or to the main Reference page? If it stays inside the p5.Vector reference page, will the page layout (a list with long description) stays the same? Not sure if this is included in the upcoming 'Reorganize the Reference group/categorization and order' task.

CC @limzykenneth @davepagurek