Describe the bug
PDF producer property is not written as expected.

To Reproduce

1. Open PDF file generated by docfx.
2. Confirm PDF document property.
3. docfx () text is displayed as producer.

Expected behavior
PDF generator exe name with version information is displayed (e.g. 'docfx (2.76.0)`)

Additional context
Currently producer property is set on following line.

But AssemblyVersionAttribute is missing on Docfx.App assembly.

It might be better to use name of Assembly.GetEntryAssembly().Name as producer name.
And use AssemblyFileVersionAttribute version instead.