Adding docs

[mishig25]

TLDR: add safetensors docs

I've added:

1. Basic structure
2. Example docstring

@Narsil please feel free to continue from here (directly push to this PR etc.)
When writing docs, please refer to this guide & transformers docs for reference

You can test the docs locally by:

pip install hf-doc-builder
pip install black watchdog
# replace the path with yours
doc-builder preview safetensors ~/Desktop/safetensors/docs/source/

Todos:

• we should probably setup.py install extras as in extras["flax"] = (jax flax); extras["testing"] = extras["flax"]+extras["torch"]+...; so that doc-builder can just call pip install .[dev]. At the moment, I hacked them in Safetensors doc-builder#323
• in the moon-ci-docs.huggingface.co preview, python setup.py develop is intalling main branch, not docs this branch. Hence,
  the docstring looks empty:

however, if you run locally with `doc-builder preview safetensors ...`, it renders as expected

I'm looking into this issue in the meantime

@Narsil please let me know of any questions 😊

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[Narsil]

LGTM