[Docs] Add spellchecking with pyenchant

- Use sphinxcontrib.spelling with custom wordlist.
- Skip the checking of the modules documents as they raise
false-positives.
- Add a setup.py spellcheck_docs command.
- Fix spelling and other issues.
- Add a doc favicon.

setup.py

@@ -443,6 +443,7 @@ cmdclass = {
     'build_trans': BuildTranslations,
     'build_plugins': BuildPlugins,
     'build_docs': BuildDoc,
+    'spellcheck_docs': BuildDoc,
     'install_data': InstallData,
     'clean_plugins': CleanPlugins,
     'clean_trans': CleanTranslations,