Quant other doc changes for relbranch pr (#27640)
* Cherry picked in changes from Jessica's branch.
Consolidate all quantization docs in quantization.rst. Add a link to quantization docs from torch.rst. Order quantization.rst alphabetically in index.rst
* Fix Quantized reference
* Add prose for Quantized Functions in the torch.nn docs
* Remove Quantization section
* Updates to index for v1.3.0
* 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/
* Doc tests caught that we'd somehow dropped documenting a few functions like
result_type, can_cast, promote_types
* Add javasphinx extension
