feat: record DocFX build version in output manifest

[vicancy]

Summary

Record the build engine's full informational version in the top-level docfx_version property of manifest.json, including prerelease and commit information when present. Read the version from the executing Docfx.Build assembly rather than the host application or Docfx.Plugins, so both CLI and library builds identify the engine that produced the output.

The stamp is applied once after manifest merging and post-processing. Templates, page metadata, and generated HTML are unchanged. Both JSON serializers use the same property name, and older manifests without it remain readable.

This follows the earlier suggestion to stamp the manifest instead of every HTML file. Motivated by comparing builds while investigating #11153; this adds diagnostics and does not fix or close the layout issue.

Validation

• All 9 DocumentBuilderTest cases pass on .NET 8 and .NET 10, including new coverage for empty, single-document, and grouped builds, both JSON serializers, older manifests, and unchanged HTML/raw models.
• A real CLI site build produces docfx_version exactly matching that CLI's --version, including the commit suffix, without stamping HTML.
• Changed-file formatting and git diff --check pass.

Local validation used an isolated checkout at da9bf7e0f with the identical changed files. The only differences from this PR's base are the subsequent Markdig 1.4.0 and Jint 4.16.3 bumps, whose packages are not yet available in the approved local feed. This PR does not change dependencies; validation against those newer versions remains for CI.