Use codespell (#504)

Author: aerosol

.codespellignore

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+Taht

.github/workflows/codespell.yml

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+name: Check spelling
+
+on:
+  pull_request:
+  push:
+    branches: [ master ]
+
+jobs:
+  codespell:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: codespell-project/actions-codespell@v2
+      with:
+        check_filenames: true
+        ignore_words_file: .codespellignore
+        path: docs