chore: deploy demo action

* create deploy demo script
* action trigger guide added

.github/workflows/ACTION_TRIGGER_GUIDE.md

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+### Actions deploy guide
+
+In order to trigger `deploy-to-demo` action you should execute the following curl:
+
+`curl -H "Accept: application/vnd.github.everest-preview+json" \
+-H "Authorization: token <ACCESS_TOKEN>" \
+--request POST \
+--data '{"event_type": "deploy-to-demo"}' \
+https://api.github.com/repos/akveo/ngx-admin/dispatches`
+
+This curl expects `ACCESS_TOKEN` input parameter
+<br>`ACCESS_TOKEN`: A personal access token. Creating a [personal access token guide](https://help.github.com/en/github/authenticating-to-github/creating-a-personal-access-token-for-the-command-line)  

.github/workflows/demoDeploy.yml

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+
+name: Deploy to demo
+
+on:
+  repository_dispatch:
+    types: deploy-to-demo
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Install SSH key
+        uses: shimataro/ssh-key-action@v2
+        with:
+          key: ${{ secrets.SSH_KEY }}
+          known_hosts: ${{ secrets.KNOWN_HOSTS }}
+      - name: Use Node.js 12.x
+        uses: actions/setup-node@v1
+      - uses: actions/checkout@v2
+        with:
+          ref: demo
+      - run: npm install
+      - run: npm run build:prod
+      - run: rsync -r --delete-after dist/. "${{ secrets.REMOTE_URL }}":"${{ secrets.ADDRESS }}"