Add a page for Boundary Desktop client

[ZedLi]

Description

This PR adds a dedicated page for the Desktop Client to the docs, and deprecates the old Desktop app page and its associated screenshots.

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GitHub issue:
RFC/PRD:

Contributor checklists

Review urgency:

• ASAP: Bug fixes, broken content, imminent releases
• 3 days: Small changes, easy reviews
• 1 week: Default expectation
• Best effort: No urgency

Pull request:

• Verify that the PR is set to merge into the correct base branch
• Verify that all status checks passed
• Verify that the preview environment deployed successfully
• Add additional reviewers if they are not part of assigned groups

Content:

• I added redirects for any moved or removed pages
• I kept this PR to one docs set and did not combine moved or renamed pages with rewrites, or I explained why in the description. Refer to Keep changes reviewable.
• I followed the Education style guide
• I looked at the local or Vercel build to make sure the content rendered correctly

[github-actions]

Vercel Previews Deployed

Name	Status	Preview	Updated (UTC)
Dev Portal	✅ Ready (Inspect)	Visit Preview	Thu Sep 17 21:34:38 UTC 2026
Unified Docs API	✅ Ready (Inspect)	Visit Preview	Thu Sep 17 21:30:22 UTC 2026

[github-actions]

Broken Link Checker

This PR contains broken links, but won't be blocked. Use this report to improve content quality:

Quick Actions

• Internal links (HashiCorp sites): Please fix these - they impact user experience
• External links: Consider if these are essential or can be updated/removed
• Temporary issues: External sites may recover - check again before merging

Need Help?

• Review our Broken Link Monitoring docs
• Check the monthly production report for context

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