docs: polish README and remove iCircuit references

[begeistert]

Description

Polishes the README and removes the iCircuit references.

• Remove iCircuit: drop the iCircuit element (RP2040Elm) — in design roadmap item.
  Generic per-pin GPIO / circuit-simulator support stays (it's a real capability).
• NuGet badges: add version + downloads badges now that the packages are published;
  make the build badge clickable.
• Installation: split "Getting Started" into a NuGet Installation section
  (dotnet add package …) and a Building from source section, leading with the
  package install.

Docs-only change; no code touched.

Type of change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 🧹 Refactoring (no functional changes, just code cleanup)

Checklist

• 🧪 Tests passed locally (dotnet test)
• ✅ Added new tests for this feature/fix
• 📝 Documentation updated (if applicable)
• 🚫 No "Magic Strings" or hardcoded values
• 💾 Validated against the RP2040 Datasheet (if applicable)