Consolidate all quantization docs in quantization.rst. Add a link to quantization docs from torch.rst. Order quantization.rst alphabetically in index.rst

* Update "Package Reference" to "Python API"
* Add in torchaudio and torchtext reference links so they show up across all docs not just the main page
* Add "Other Languages" section, add in C++ docs, add in Javadocs
* Add link to XLA docs under Notes: http://pytorch.org/xla/

…s like

result_type, can_cast, promote_types