Add profile-stack example

[chacha912]

What this PR does / why we need it?

• Add profile-stack example

• When multiple users access the profile-stack example page, the number of users does not match the actual number of users who access the document because the document key is the same. Would it be better to access the example page with a new document key?
  • In the example below, two users are displayed on the page, but four are displayed in the iframe.
    

Any background context you want to provide?

What are the relevant tickets?

Fixes #

Checklist

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

[hackerwins]

Thanks for your contribution.

[hackerwins]

It seems that this file is empty because Profile Stack doesn't have a document structure.

How about changing DocumentStructure to How it works and explaining how it behaves?
For that, it might be better to express it in markdown format.

[hackerwins]

When multiple users access the profile-stack example page, the number of users does not match the actual number of users who access the document because the document key is the same. Would it be better to access the example page with a new document key?

It would be better to issue a new document key. For the same reason, I think liveblocks also issues keys(roomId) for each user.