Problem

The need to have meaningful and actionable content rendered on empty views has been arising across the workbench:

• SCM: SCM: Viewlet should improve "no repositories" messaging #61491
• Debug: Make run and debug welcome view extensible #85548
• Custom Views: Remote Containers: Guide users to create the first container when non exists vscode-remote-release#1349

Proposal

Let's enable the following in the core:

1. Add an overridable isEmpty(): boolean method to ViewPane which would let view panes dictate when they are empty, as well as onDidChangeEmpty event
2. Create a core contribution point for empty view content, supporting very simple Markdown (links only)
3. Enhance the simple Markdown to be able to detect isolated command links and render them as buttons
4. Render all empty view contributions in a ViewPane when isEmpty() === true

We can then update our API by exposing an extension contribution point for the same core contribution point created above. Here's a couple of proposals:

contributes.documentation.VIEWID

"contributes": {
  "documentation": {
    "VIEWID": [{
       "label": "MARKDOWN",
       "when": "CONDITION"
    }]
  }
}

Alternatives for documentation: viewhelp, emptyview.

Alternatives for label: text, markdown, description.

contributes.documentation.view

"contributes": {
  "documentation": {
    "view": [{
       "label": "MARKDOWN",
       "when": "VIEWID && CONDITION"
    }]
  }
}

cc @sbatten for views
cc @isidorn for debug
cc @sandy081, @alexr00 for custom views
cc @bowdenk7 for scm
cc @jrieken for input on API conventions