[migration] Make package README repository links portable #4989

Parent: #4960
Depends on: portable repository identity foundation

Extract package metadata/README portability from #4964.

Scope

• Centralize current repository and documentation URLs as deterministic MSBuild properties with explicit override and offline fallback.
• Parameterize generated and custom SkiaSharp/HarfBuzz/Views package README templates.
• Preserve stable package fwlinks, package IDs, public API, and current mono links until cutover.
• Validate every changed package template under mono and simulated dotnet values, including XML/HTML/special-character round trips.
• Ensure identity inputs participate in managed/package cache keys.

Exclusions

• No package publication, NuGet ID/version changes, Pages deployment, Gallery/DocFX work, or Backport.

Completion

• New packages require changing one centralized identity/site value rather than editing multiple README files.