[ci] add a couple gh actions

1. setup automerging for specially labeled PRs
2. add link checking in markdown documents
3. prohibit merging PRs with auto-squashable commits

.github/workflows/automerge.yml

+name: automerge
+on:
+  pull_request:
+    types:
+      - labeled
+      - unlabeled
+      - synchronize
+      - opened
+      - edited
+      - ready_for_review
+      - reopened
+      - unlocked
+  pull_request_review:
+    types:
+      - submitted
+  check_suite: 
+    types:
+      - completed
+  status: {}
+jobs:
+  automerge:
+    runs-on: ubuntu-latest
+    steps:
+      - name: automerge
+        uses: "pascalgn/automerge-action@135f0bdb927d9807b5446f7ca9ecc2c51de03c4a"
+        env:
+          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+          UPDATE_METHOD: "rebase"
+          MERGE_METHOD: "rebase"
+          UPDATE_LABELS: "automerge"
+          MERGE_LABELS: "automerge"
+          MERGE_FORKS: "false"
+          MERGE_DELETE_BRANCH: "true"

.github/workflows/autosquash_check.yml

+on: pull_request
+
+name: Pull Requests
+
+jobs:
+  message-check:
+    name: Block Autosquash Commits
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Block Autosquash Commits
+        uses: xt0rted/block-autosquash-commits-action@master
+        with:
+          repo-token: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/link_check.yml

+# homepage: https://github.com/gaurav-nelson/github-action-markdown-link-check
+name: Check Markdown links
+
+on: [pull_request]
+
+jobs:
+  markdown-link-check:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: gaurav-nelson/github-action-markdown-link-check@v1