Revert to netstandard2.0 only — preserve cross-platform design

The point of this fork is cross-platform Excel generation without
a Windows-only dependency. Targeting net8.0-windows / net9.0-windows
defeated that purpose by making modern .NET explicitly Windows-only.

- Change TargetFrameworks → TargetFramework: netstandard2.0
- Merge per-TFM shim ItemGroup into single unconditional group
- Simplify CI/CD workflows: ubuntu-latest runner (no -windows needed)
- Update README and docs: drop Windows-only framework table rows
- Bump version to 2.0.1

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Migrate to .NET Standard 2.0 / .NET 8+9 (Windows); add CI/CD and GitH…

…ub Pages

- Switch SDK from Microsoft.NET.Sdk.Web to Microsoft.NET.Sdk
- Multi-target netstandard2.0;net8.0-windows;net9.0-windows
- Replace abandoned CoreCompat.System.Drawing.v2 with System.Drawing.Common 9.0.0
- Upgrade System.Security.Cryptography.Pkcs, System.Text.Encoding.CodePages to 9.0.0
- Fix CS0579 duplicate assembly attributes in Properties/AssemblyInfo.cs
- Fix CS0104 ambiguous Index vs System.Index with using alias
- Fix CS1069 SecurityPermission unavailable on net5+ via #if NET5_0_OR_GREATER
- Add .github/workflows/build.yml (CI on every push/PR)
- Add .github/workflows/publish.yml (NuGet + GitHub Release on version tag)
- Add docs/index.html Bootstrap 5.3 GitHub Pages landing page
- Rewrite README.md with ASCII art, badges, and modern install/usage docs

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>