Describe .archify (YAML) as the diagram format everywhere (0.5.1)

- Marketplace description, README and messages lead with .archify files
  in YAML; Archify's JSON files and rendered HTML are described as also
  supported, in their own README section and a supported-files table
- Rename the rendered-HTML editor to Archify HTML Viewer (it shared the
  extension's name) and its command to Open in Archify HTML Viewer
- Empty or broken .archify files ask for valid YAML instead of JSON

src/preview.ts

@@ -1,6 +1,6 @@
 import path from 'node:path';
 import * as vscode from 'vscode';
-import { readDiagramSource } from './detect';
+import { isArchifyFile, readDiagramSource } from './detect';
 import { renderDiagram } from './renderer';
 import { evidenceMode, log, readText, runtimeFor, viewerTheme } from './runtime';
 import { handleWebviewMessage, WebviewMessage } from './webviewMessages';
@@ -100,7 +100,7 @@ class DiagramPreview implements vscode.Disposable {
     if (!source) {
       this.showMessage(
         'Nothing to show yet',
-        'Set "diagram_type" to architecture, workflow, sequence, dataflow or lifecycle, and make sure the file is valid JSON.',
+        `Set "diagram_type" to architecture, workflow, sequence, dataflow or lifecycle, and make sure the file is valid ${isArchifyFile(this.uri.path) ? 'YAML' : 'JSON'}.`,
       );
       return;
     }