Improve post-launch activation and document CI onboarding

[jinyounghub]

Summary

• move a verified one-shot dnx demo into the first README viewport and add Marketplace/contract entry points
• add a conversion-focused GitHub Pages landing page while keeping the offline report under /report/
• add a generic observation-mode consumer workflow plus a live synthetic PDF contract
• expose uploaded Action artifact links in job summaries and validate their outputs
• add custom social-preview and package-icon assets, with version-drift checks
• clarify CI artifact confidentiality and improve mobile, high-contrast, and screen-reader behavior

Validation

• dnx DocRefract.Tool@0.2.3 -- demo --out ... → content 1, format 1, layout 1
• Release build: 0 warnings, 0 errors
• Tests: 29 passed
• packaged tool smoke contract passed (exit codes 0/1/2)
• NuGet archive contains docrefract-icon.png and matching nuspec metadata
• assembled Pages required-file and internal-link checks passed
• all Action/workflow YAML parsed successfully
• verify-version-sync.ps1 passed
• independent technical and responsive/accessibility reviews: no remaining findings

No community posts or third-party outreach are included.