Add docs on how to automate publishing

[reduckted]

A follow-up from here: VsixCommunity/Community.VisualStudio.Toolkit#267 (comment)

Some possible topics to cover:

• How to set up a build pipeline for:
  • GitHub Actions
  • Azure Pipelines?
  • AppVeyor?
• What to define in a publish.json manifest file (there's already good docs on this here, so maybe all we need to do is provide a link to that page.
• How to create a personal access token (there's some docs on that here, so again, we may not need to document too much on this).