Update dashboard.md - Add documentation for hiding and restricting dashboard access

[steveatkiss]

📋 Description

Adds a new section documenting how to hide dashboards or add conditional access rules using the extension registry.

This addresses a common gap in the documentation where developers need to:

• Hide default dashboards like the Getting Started dashboard
• Restrict dashboard access to specific user groups (e.g., admins only)
• Use extensionRegistry.exclude() and extensionRegistry.appendCondition()

Includes examples for:

• Removing dashboards completely
• Restricting to admin users only
• Restricting to multiple specific user groups using GUIDs

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• [ x ] Code blocks are correctly formatted.
• [ x ] Sentences are short and clear (preferably under 25 words).
• [ x ] Passive voice and first-person language (“we”, “I”) are avoided.
• [ x ] Relevant pages are linked.
• [ x ] All links work and point to the correct resources.
• [ x ] Screenshots or diagrams are included if useful.
• [ x ] Any code examples or instructions have been tested.
• [ x ] Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

v16

[eshanrnh]

Thanks @steveatkiss for the PR 🙌 We will review it soon.

[iOvergaard]

Added a few changes to entry point type and added a link to the condition article.

[eshanrnh]

Looks great, @steveatkiss 🙌

[steveatkiss]

That's great, thanks for tidying and getting in place, hopeufully will be helpful for some people.

[eshanrnh]

Absolutely, @steveatkiss. Every contribution is important 🙌