chore: prepare repo for HACS listing

- Add hacs.json and MIT LICENSE required for HACS/default-store inclusion
- Fill required manifest.json keys (documentation, issue_tracker,
  codeowners) and reorder per hassfest's key-ordering rule
- Add validate.yml workflow running hassfest and HACS validation
- Update README install note now that hacs.json is checked in

.github/workflows/validate.yml

@@ -0,0 +1,40 @@
+name: Validate
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: "0 0 * * *"
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+permissions: {}
+
+jobs:
+  hassfest:
+    name: Hassfest validation
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout the repository
+        uses: actions/checkout@v4
+
+      - name: Run hassfest validation
+        uses: home-assistant/actions/hassfest@master
+
+  hacs:
+    name: HACS validation
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout the repository
+        uses: actions/checkout@v4
+
+      - name: Run HACS validation
+        uses: hacs/action@main
+        with:
+          category: integration
+          # Remove this 'ignore' key once real brand images are added at
+          # custom_components/localthings/brand/
+          ignore: brands