Update k-NN documentation for faiss support feature

[jmazanec15]

Description

This PR adds documentation update for k-NN's faiss support feature. For more information on faiss feature, please checkout the RFC. This feature is for 1.2.

Overall, the major changes are that:

1. We added support for the faiss library and some of its ANN algorithms
2. We added support for ANN algorithms that require training through the concept of models
3. Switched reference of graphs to native library indices

With this, we added a few additional stats and settings.

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