Spelling corrections plus CI job for codespell

* Use codespell to check spelling in documentation, refs #1417
* Fixed spelling errors spotted by codespell, closes #1417
* Make codespell a docs dependency

See also this TIL:  https://til.simonwillison.net/python/codespell

.github/workflows/spellcheck.yml

@@ -0,0 +1,25 @@
+name: Check spelling in documentation
+
+on: [push, pull_request]
+
+jobs:
+  spellcheck:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v2
+      with:
+        python-version: 3.9
+    - uses: actions/cache@v2
+      name: Configure pip caching
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/setup.py') }}
+        restore-keys: |
+          ${{ runner.os }}-pip-
+    - name: Install dependencies
+      run: |
+        pip install -e '.[docs]'
+    - name: Check spelling
+      run: codespell docs/*.rst --ignore-words docs/codespell-ignore-words.txt