Add GH Actions workflow to create an issue on changed usage part of imgproxy docs

Author: DarthSim

.github/templates/ISSUE.md

@@ -0,0 +1,7 @@
+---
+title: Usage docs of imgproxy have been updated
+assignees: DarthSim
+labels: enhancement
+---
+@{{ payload.client_payload.actor }} has just updated the [Usage](https://docs.imgproxy.net/usage) part of the documentation.
+Please, check [the difference]({{ payload.client_payload.link }}) and make the necessary changes.

.github/workflows/imgproxy-usage-updated.yml

@@ -0,0 +1,28 @@
+#
+# This action is triggered by the repository_dispatch event
+# published by the imgproxy/imgproxy-docs repository.
+# It creates an issue and assign it to the gem's maintainers.
+#
+---
+name: Create an issue about updated imgproxy docs
+on:
+  repository_dispatch:
+    types:
+      - imgproxy-usage-updated
+
+jobs:
+  create-issue:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      issues: write
+    steps:
+      - name: Checkout the repository
+        uses: actions/checkout@v4
+
+      - name: Create an issue
+        uses: JasonEtco/create-an-issue@v2
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          filename: .github/templates/ISSUE.md