feat: rendering improvements and fullscreen mermaid diagrams

[MohamedH1998]

Summary

• Adds a click-to-expand dialog for mermaid diagrams — hover to reveal the button, click to view the full diagram (with annotation footer and branding) in a full-viewport overlay. Animated open/close, keyboard/touch accessible.
• Switches to linear edge curves, muted edge label text, and dynamic --sl-color-bg backgrounds on labels so they match the page exactly in both themes.
• Skips all mermaid setup (including the ~2.5 MB bundle import) on pages without diagrams. Lazy-creates the dialog on first use. Adds per-diagram error handling so one broken diagram doesn't take out the rest.

Screenshots (optional)

Before

After

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
*.ts	@cloudflare/content-engineering, @kodster28
*	@cloudflare/pcx-technical-writing, @cloudflare/product-owners

[github-actions]

Preview URL: https://09c74d8b.preview.developers.cloudflare.com
Preview Branch URL: https://feat-tweak-mermaid-diagram.preview.developers.cloudflare.com