Simplify and update package README.md files for stable release

[Copilot]

Simplifies the package READMEs for ModelContextProtocol.Core, ModelContextProtocol, and ModelContextProtocol.AspNetCore to match the pattern established in #1387 for the root README. Consolidates all three per-package READMEs into a single shared src/PACKAGE.md that is packed as README.md into each NuGet package. Also updates the top-level README.md with consistent documentation links.

• Created src/PACKAGE.md as the single shared NuGet package README with simplified content, Packages section with absolute docs site URLs, Getting Started link, and "Official MCP Documentation" / "MCP C# SDK Documentation" links
• Deleted individual src/ModelContextProtocol/README.md, src/ModelContextProtocol.Core/README.md, and src/ModelContextProtocol.AspNetCore/README.md files
• Updated all three .csproj files to reference ../PACKAGE.md and pack it as README.md inside the NuGet package
• Updated top-level README.md: renamed "Official Documentation" to "Official MCP Documentation", added "MCP C# SDK Documentation" link, and changed package Documentation links to use absolute docs site URLs

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[jeffhandley]

@copilot Review the changes made in #1387 and apply similar editing here, simplifying the package READMEs in the same manner. Instead of including Getting Started content with samples in the package README files, link to the Getting Started documentation page introduced and referenced in that PR. Also look change "our API documentation" to "the API documentation" as was done in the other PR.

[Copilot]

@copilot Review the changes made in #1387 and apply similar editing here, simplifying the package READMEs in the same manner. Instead of including Getting Started content with samples in the package R...

Simplified all three package READMEs to match the pattern from #1387 in 9382bb5. Removed all inline code samples and replaced Getting Started sections with links to the documentation page. Changed "our API documentation" to "the API documentation" across all files. Also tightened each package's description to be more specific about what it provides.

[stephentoub]

@jeffhandley at this point I'm not sure why we have the READMEs in each library folder. Can we just delete those and have the nuget package for each just include the main readme?

[jeffhandley]

@stephentoub yeah, great point. Sounds good! What we've done in dotnet/runtime is name the package readme as PACKAGE.md, which has also been helpful for us to recognize which is when editing. So I suggest:

1. Create a PACKAGE.md that sits in the src folder
2. Have all 3 packages pull that file in and rename it to README.md inside the package

In dotnet/runtime, we use some arcade/common build infrastructure to achieve that. Here, it should be something like this:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <!-- NuGet will look for this path *inside the nupkg* -->
    <PackageReadmeFile>README.md</PackageReadmeFile>
  </PropertyGroup>

  <ItemGroup>
    <!-- Take PACKAGE.md from the repo and pack it as README.md at the nupkg root -->
    <None Include="PACKAGE.md"
          Pack="true"
          PackagePath="README.md"
          PackageCopyToOutput="true" />
  </ItemGroup>

</Project>

[stephentoub]

@stephentoub yeah, great point. Sounds good! What we've done in dotnet/runtime is name the package readme as PACKAGE.md, which has also been helpful for us to recognize which is when editing. So I suggest:

1. Create a PACKAGE.md that sits in the src folder
2. Have all 3 packages pull that file in and rename it to README.md inside the package

In dotnet/runtime, we use some arcade/common build infrastructure to achieve that. Here, it should be something like this:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <!-- NuGet will look for this path *inside the nupkg* -->
    <PackageReadmeFile>README.md</PackageReadmeFile>
  </PropertyGroup>

  <ItemGroup>
    <!-- Take PACKAGE.md from the repo and pack it as README.md at the nupkg root -->
    <None Include="PACKAGE.md"
          Pack="true"
          PackagePath="README.md"
          PackageCopyToOutput="true" />
  </ItemGroup>

</Project>

@copilot, please do this

[Copilot]

@stephentoub yeah, great point. Sounds good! What we've done in dotnet/runtime is name the package readme as PACKAGE.md, which has also been helpful for us to recognize which is when editing. So I s...

Done in 91756aa. Created src/PACKAGE.md as the shared NuGet package README and updated all three csproj files to reference it via <None Include="..\PACKAGE.md" Pack="true" PackagePath="\README.md" />. Deleted the individual README.md files from each package folder. Also updated the top-level README.md Documentation links to use absolute docs site URLs since the per-package READMEs no longer exist.