Add new tutorial: #1 Embedding p5.js with an iframe

[hxrshxz]

Addresses #999

Description

Adds new tutorial "Embedding p5.js with an iframe" to the 2.0 branch and fixes a bug in TutorialLayout.

Changes

• New tutorial: embedding-p5-with-iframe.mdx - Comprehensive guide on embedding p5.js sketches using iframes
• New category: Added "beyond-web-editor" to tutorials config
• Bug fix: Added optional chaining in TutorialLayout.astro for optional featuredImage and authors fields to prevent crashes

Tutorial Details

• Category: beyond-web-editor
• Authors: hxrshxz, Dave Pagurek
• Covers: iframe basics, global mode, multiple sketches, security, responsive design, and best practices

The optional chaining fix prevents TypeError when tutorials don't include the optional featuredImage or authors frontmatter fields.