Adding usage examples for common tasks (#2850)

* Usage: Sequence Classification & Question Answering

* Pipeline example

* Language modeling

* TensorFlow code for Sequence classification

* Custom TF/PT toggler in docs

* QA + LM for TensorFlow

* Finish Usage for both PyTorch and TensorFlow

* Addressing Julien's comments

* More assertive

* cleanup

* Favicon
- added favicon option in conf.py along with the favicon image
- udpated 🤗 logo. slightly smaller and should appear more consistent across editing programs (no more tongue on the outside of the mouth)

Co-authored-by: joshchagani <joshua@joshuachagani.com>

docs/source/index.rst

@@ -61,6 +61,7 @@ The library currently contains PyTorch and Tensorflow implementations, pre-train
     quickstart
     glossary
     pretrained_models
+    usage
     model_sharing
     examples
     notebooks