Add ProseMirror example to documentation site

[hackerwins]

What this PR does / why we need it?

Add a new ProseMirror collaborative editing example page following the existing CodeMirror/Quill pattern, using the @yorkie-js/prosemirror binding and the Tree CRDT type.

Any background context you want to provide?

What are the relevant tickets?

Fixes #

Checklist

• Added relevant tests or not required
• Didn't break anything

Summary by CodeRabbit

• New Features
  • Added a new ProseMirror collaborative editor example showcasing real-time collaboration capabilities with Yorkie's Tree CRDT type.
  • The example includes a live preview, code viewer, and comprehensive guide documentation.

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR adds a new vanilla ProseMirror collaborative editor example to the codebase. It includes the example file descriptor, a new Next.js page component for displaying the example, updates to the example registry, and modifications to build and fetch scripts to incorporate the new example.

Changes

Cohort / File(s)	Summary
Example File Structure examples/vanilla-prosemirror/fileInfo.ts	New file exporting FILE_INFO constant describing the directory structure and content of the vanilla-prosemirror example for programmatic access.
Example Pages pages/examples/index.tsx, pages/examples/prosemirror.tsx	Added new ProseMirror example entry to the EXAMPLES registry and created a new Next.js page component that renders the example with sidebar, code viewer, and live preview iframe using ExampleLayout utilities.
Build and Fetch Scripts scripts/build-js-examples.mts, scripts/fetch-js-examples.mts	Updated build script to include 'vanilla-prosemirror' in STATIC_EXAMPLES list and extended package.json regex to handle prosemirror package versioning alongside existing sdk and react packages.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

🐰 A new example hops into the pen,
With ProseMirror dancing once again,
File info mapped, registry fed,
Scripts updated with prosemirror thread,
Collaboration blooms in every spread! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main objective of the pull request: adding a ProseMirror example to the documentation site.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch prosemirror

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