Support custom assets in the overview.md

[dnikolovv]

If any custom local images are referenced in the given overview.md they should be included in the publishManifest.json and copied to the VSIX project directory.

See https://docs.microsoft.com/en-us/visualstudio/extensibility/walkthrough-publishing-a-visual-studio-extension-via-command-line?view=vs-2019#asset-files