Add package readmes to illink NuGet packages stating SDK-only support (#123505)

## Description

The `Microsoft.NET.ILLink` and `Microsoft.NET.ILLink.Tasks` NuGet
packages are published but lack documentation indicating they're
SDK-internal and unsupported for direct consumption.

## Changes

- **Added `PACKAGE.md` files** to both packages with clear "Do not
directly reference this package" language matching other SDK-internal
packages like `Microsoft.NETCore.App`
- **Enabled package readme inclusion** by removing
`EnableDefaultPackageReadmeFile=false` from `ILLink.Tasks.csproj` and
adding `EnableDefaultPackageReadmeFile=true` to `Mono.Linker.csproj`
- **Included trimming documentation links** for users who need guidance
on proper trimming usage through the SDK

Both readmes follow the standard repository PACKAGE.md structure with
About, How to Use, Additional Documentation, and Feedback sections.

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Illink nuget packages should state support contract in
package readme</issue_title>
> <issue_description>The illink packages aren’t supported outside of use
by the SDK. We should state that in the readme. </issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes #123504

<!-- START COPILOT CODING AGENT TIPS -->
---

💡 You can make Copilot smarter by setting up custom instructions,
customizing its development environment and configuring Model Context
Protocol (MCP) servers. Learn more [Copilot coding agent
tips](https://gh.io/copilot-coding-agent-tips) in the docs.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: agocke <515774+agocke@users.noreply.github.com>

Author: Copilot

src/tools/illink/src/ILLink.Tasks/ILLink.Tasks.csproj

@@ -18,8 +18,6 @@
     <IncludeBuildOutput>false</IncludeBuildOutput>
     <IncludeMultiTargetRoslynComponentTargets>false</IncludeMultiTargetRoslynComponentTargets>
     <TargetsForTfmSpecificContentInPackage>$(TargetsForTfmSpecificContentInPackage);AddBuildOutputToToolsPackage</TargetsForTfmSpecificContentInPackage>
-    <!-- TODO: Add package readme -->
-    <EnableDefaultPackageReadmeFile>false</EnableDefaultPackageReadmeFile>
     <!-- Don't use the TFM but a short form of it without the version to simulate the package layout. -->
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
     <OutputPath Condition="'$(TargetFramework)' == '$(NetCoreAppToolCurrent)'">$(OutputPath)net\</OutputPath>

src/tools/illink/src/ILLink.Tasks/PACKAGE.md

@@ -0,0 +1,17 @@
+## About
+
+MSBuild tasks for running the IL Linker to trim .NET applications.
+
+## How to Use
+
+Do not directly reference this package. It is used internally by the .NET SDK when projects enable trimming.
+
+## Additional Documentation
+
+- [Trim self-contained deployments and executables](https://learn.microsoft.com/dotnet/core/deploying/trimming/trim-self-contained)
+- [Introduction to trim warnings](https://learn.microsoft.com/dotnet/core/deploying/trimming/fixing-warnings)
+- [Prepare .NET libraries for trimming](https://learn.microsoft.com/dotnet/core/deploying/trimming/prepare-libraries-for-trimming)
+
+## Feedback & Contributing
+
+Microsoft.NET.ILLink.Tasks is released as open source under the [MIT license](https://licenses.nuget.org/MIT). Bug reports and contributions are welcome at [the GitHub repository](https://github.com/dotnet/runtime).

src/tools/illink/src/linker/Mono.Linker.csproj

@@ -26,6 +26,7 @@
     <TargetsForTfmSpecificContentInPackage>$(TargetsForTfmSpecificContentInPackage);_AddReferenceAssemblyToPackage</TargetsForTfmSpecificContentInPackage>
     <DefineConstants>$(DefineConstants);ILLINK</DefineConstants>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <EnableDefaultPackageReadmeFile>true</EnableDefaultPackageReadmeFile>
   </PropertyGroup>
 
   <Target Name="_ComputePackRuntimeConfigFilePath">

src/tools/illink/src/linker/PACKAGE.md

@@ -0,0 +1,17 @@
+## About
+
+IL Linker tool for trimming .NET applications.
+
+## How to Use
+
+Do not directly reference this package. It is used internally by the .NET SDK when projects enable trimming.
+
+## Additional Documentation
+
+- [Trim self-contained deployments and executables](https://learn.microsoft.com/dotnet/core/deploying/trimming/trim-self-contained)
+- [Introduction to trim warnings](https://learn.microsoft.com/dotnet/core/deploying/trimming/fixing-warnings)
+- [Prepare .NET libraries for trimming](https://learn.microsoft.com/dotnet/core/deploying/trimming/prepare-libraries-for-trimming)
+
+## Feedback & Contributing
+
+Microsoft.NET.ILLink is released as open source under the [MIT license](https://licenses.nuget.org/MIT). Bug reports and contributions are welcome at [the GitHub repository](https://github.com/dotnet/runtime).