Add links to Javadocs in reference pages

[SableRaf]

Which area of the website does this enhancement relate to?

Reference

Describe the enhancement

In the previous version of the Processing website, each reference page linked to the relevant class or method in the Javadocs. These links are missing from the current site.

Why is this enhancement important?

Javadoc links are valuable for interested users who need more detailed information about the Processing API.

Additional context

Javadocs for Processing 4 are automatically generated at https://github.com/processing/processing4-javadocs

They are hosted on GitHub pages at https://processing.github.io/processing4-javadocs/