Building doc languageLoader.js from conf.py
This eliminates the hardcoded language codes from languageLoader.js, make it a template and build the real deal from conf.py. So, whenever sphinx-build is invoked, the languageLoader.js is created.
This also adds doc/_static/languageLoader.js to .gitignore so it is only created and used in real time when building documentation.
This was inspired by Python documentation's build_docs.py solution to generate the (version x language) switcher.js.