Fill in template contents for explainer repositories. (#1)

This includes some differences from the TAG's template explainer and the WICG starter kit's template bikeshed file, which I'll try to upstream next.

.github/workflows/auto-publish.yml

@@ -0,0 +1,17 @@
+name: CI
+on:
+  pull_request: {}
+  push:
+    branches: [main]
+jobs:
+  main:
+    name: Build, Validate and Deploy
+    runs-on: ubuntu-22.04
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v4
+      - uses: w3c/spec-prod@v2
+        with:
+          GH_PAGES_BRANCH: gh-pages
+          BUILD_FAIL_ON: warning

.gitignore

@@ -0,0 +1 @@
+index.html

.pr-preview.json

@@ -0,0 +1,8 @@
+{
+  "src_file": "index.bs",
+  "type": "bikeshed",
+  "params": {
+    "force": 1
+  }
+}
+