fix: replace beta.p5js.org links with p5js.org in contributor docs

[Nwakaego-Ego]

Addresses processing/p5.js-website#1493

Replaces beta.p5js.org links with p5js.org in contributor docs following the p5.js v2 website switch.

• contributor_docs/contributing_to_the_p5js_reference.md: updated all beta.p5js.org links to p5js.org. Also updated line 33 phrasing from "currently being hosted at beta.p5js.org/reference/" to "is at p5js.org/reference/" to reflect that v2 is now the live site.
• contributor_docs/p5.strands.md: updated beta.p5js.org links to p5js.org
• contributor_docs/steward_guidelines.md: updated beta.p5js.org links to p5js.org
• contributor_docs/working_with_contributor_documents.md: updated beta.p5js.org to p5js.org and updated the v1 link from p5js.org to v1.p5js.org to correctly reflect the new subdomain.
• contributor_docs/es/steward_guidelines.md: updated beta.p5js.org links to p5js.org

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