Added spellcheck utility to documentation build

Uses the sphinxcontrib-spelling extension and requires PyEnchant and
aspell-en to be installed.

Adds the optional make target 'spelling' which produces the file
spelling/output.txt. It contains all words it detects.

Due to the large number of false positives, words in
utils/sphinx-config/false_positives.txt are ignored.

doc/utils/sphinx-config/conf.py

@@ -299,4 +299,11 @@ more information about the code and its uses.
 
 epub_author = 'The LAMMPS Developers'
 
+# configure spelling extension if present
+import importlib.util
+spelling_spec = importlib.util.find_spec("sphinxcontrib.spelling")
+if spelling_spec:
+    extensions.append('sphinxcontrib.spelling')
 
+    spelling_lang='en_US'
+    spelling_word_list_filename='false_positives.txt'