Port Docs-Viewer to slatehtml as the docs-viewer package.

Replace the classic DOM shell with slate widgets, markdown→slate rendering, and a CI index build that uses build-docs.cjs.

.github/workflows/main.yml

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - master
+      - slatehtml
 jobs:
   build:
     runs-on: ubuntu-latest
@@ -11,15 +12,15 @@ jobs:
     permissions:
       contents: write
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       - name: Setup Node.js
-        uses: actions/setup-node@v3
+        uses: actions/setup-node@v4
         with:
-          node-version: '18'
+          node-version: "22"
 
-      - name: Build docs
-        run: node build-docs.js
+      - name: Build docs index
+        run: node build-docs.cjs
 
       - name: Check for changes
         id: check_changes