Address PR review feedback and lint VPN documentation

[Copilot]

Addressed 9 review comments on the VPN setup documentation, fixing clarity issues, adding security warnings, and ensuring markdown compliance.

Structural improvements:

• Added explicit installation step (step 3) between download and DNS configuration
• Reorganized steps so FortiClient installation precedes configuration file download
• Fixed image path from assets/ to ../assets/images/ for correct relative resolution

Security and warnings:

• Added warning box for DNS configuration explaining permanent system-wide changes with revert command
• Added security notice for "Save login" feature, advising use only on trusted, non-shared computers

Clarity enhancements:

• Specified UI element locations: "three-bar menu icon (usually in the top-right corner)"
• Clarified interface components: "XML tab", "+ Import XML Configuration button"
• Improved phrasing: "Click the three-bar menu icon again" (was "Hit again the three bars")
• Corrected vocabulary: "enter your password" (was "introduce your password")

Linting:

• Fixed MD013 line-length violations (120 character limit)
• All markdownlint-cli2 checks now pass

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