Developer experience improvements: fix README, add missing fluent extensions, correct package description

[Copilot]

The README had a copy-paste bug (bookmark section showed identical code to the template-open section), lacked any Report Engine documentation, and the NuGet package description incorrectly identified this as a "MvvmCross plugin". Two ContextModel data types (FileLinkModel, SubstitutableStringModel) also had no fluent extension methods despite the rest of the API following that pattern.

Changes

• README.md – Full rewrite:

  • Fixed bookmark section with working SetTextOnBookmark / SetTextsOnBookmark / SetHtmlOnBookmark examples
  • Added Report Engine usage docs: simple document, data-driven table from collection, multi-report generation, stream-based template opening
  • Added fluent extension reference table

• ContextModelExtensions.cs – Two missing helpers:

  • AddFileLink(key, filePath) — wraps FileLinkModel (image/file path binding)
  • AddSubstitutableString(key, pattern, dataSource) — wraps SubstitutableStringModel (composite formatted strings)

  var context = new ContextModel()
      .AddString("#CustomerName#", "Alice")
      .AddFileLink("#SignaturePath#", @"C:\assets\signature.png")
      .AddSubstitutableString("#Summary#", "{0} orders totalling {1}",
          new ContextModel()
              .AddString("0", "42")
              .AddDouble("1", 1234.5, "{0:C2}"));

• OpenXMLSDK.Engine.csproj – Corrected <Description> to accurately describe the library.