Clone p5.js's latest release instead of main, and remove old reference items

[davepagurek]

Resolves #522

This makes two changes to builder scripts:

• A little update to make it easier to make updates to the reference builder (the thing npm run build:reference calls.) Currently, it just clones the p5.js repo's main branch, which is ever changing, but we only want to publish the current release's info on the site. Now, we can rebuild the reference as much as we want without worrying about "leaking" new APIs before they're released.
• Makes it remove all the reference items before building again. Previously, it would overwrite existing files or create new files where they do not yet exist, but it would never delete things that are no longer in the reference. This resolves Problem with html symbols at https://p5js.org/reference/#Foundation #522 because that item simply is no longer in the reference.