GHA: add a job scanning for "bad words" in markdown

This means words, phrases or things we have decided not to use - words that
are spelled right according to the dictionary but we want to avoid. In the
name of consistency and better documentation.

Closes #12764

.github/workflows/badwords.yml

@@ -0,0 +1,27 @@
+# Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+#
+# SPDX-License-Identifier: curl
+
+name: badwords
+
+on:
+  # Trigger the workflow on push or pull requests, but only for the
+  # master branch
+  push:
+    branches:
+    - master
+    - '*/ci'
+  pull_request:
+    branches:
+    - master
+
+jobs:
+  check:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: check
+      run: ./.github/scripts/badwords.pl < .github/scripts/badwords.txt docs/*.md docs/libcurl/*.md docs/libcurl/opts/*.md