feat: add VSIX release metadata

[rcappello]

This PR prepared the Visual Studio 2026 package metadata for the next functional release without changing the current version or triggering a release.

Changes

• Added release notes, icon, and preview image metadata to the VSIX manifest.
• Kept the release-notes URL aligned with the metadata generated from ExtensionEntrypoint.cs.
• Added a release instruction requiring future VS 2026 versions to synchronize the release URL, manifest version, and tag.
• Restored the required description in the instruction file frontmatter.

Related Issues

None.

Notes

• No release is required for this change alone; the metadata is ready to ship with the next functional VS 2026 release.
• dotnet build src/vs2026/PipelinesExplorer.VisualStudio.csproj -c Release passed, and the generated VSIX retained the new metadata and packaged assets.