Document ImageSharp 3.x pin (avoid 4.0 license-key requirement)

ImageSharp 4.0 enforces the Six Labors Split License and fails the build
without a registered license key. Add a comment pinning the dependency
to the Apache-2.0 3.1.x line so the zero-config OSS build keeps working
and the upgrade isn't re-attempted.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Netdocs

Directory.Packages.props

@@ -18,6 +18,9 @@
     <PackageVersion Include="Microsoft.AspNetCore.WebSockets" Version="2.3.0" />
     <PackageVersion Include="AngleSharp" Version="1.5.2" />
     <PackageVersion Include="LibGit2Sharp" Version="0.31.0" />
+    <!-- Pinned to the 3.x line on purpose: ImageSharp 4.0 switched to the Six Labors Split
+         License and fails the build without a registered license key (sixlabors.lic). Netdocs is
+         a zero-config OSS tool distributed to users, so we stay on the Apache-2.0 3.1.x releases. -->
     <PackageVersion Include="SixLabors.ImageSharp" Version="3.1.12" />
     <PackageVersion Include="SixLabors.ImageSharp.Drawing" Version="2.1.7" />
     <PackageVersion Include="xunit" Version="2.9.3" />