Add static docs

[arnaudmiribel]

Summary

Solves https://github.com/arnaudmiribel/streamlit-extras/issues/173

Our gallery is down sometimes, it'd be great to have a static docs backup at least, or maybe even turn that into the default. This PR introduces new static docs [preview] based off material-mkdocs

Content

1. General config for mkdocs-material. Includes a set of plugins to easily annotate docstrings or highlight, color code blocks
2. A GitHub action to make it possible to publish and update docs after every PR
3. A workflow to publish preview docs into Netlify
4. A few edits of our extras docstrings so they can be parsed properly with mkdocstrings
5. Embedded stlite iframes
6. Auto-resize of those iframes

How does mkdocs work?

Locally

1. Install docs requirements

   pip install -r docs/requirements.txt

2. Now serve the docs! It should load in http://127.0.0.1:8000

   mkdocs serve

Publishing

With the following command (use with caution), the docs will get published at https://arnaudmiribel.github.io/streamlit-extras/ - automatically building in the gh-deploy branch and reading from there, which is the default GitHub pages looks into.

mkdocs gh-deploy --force

[blackary]

You should indeed add that you need to do pip install -r docs/requirements.txt, because otherwise mkdocs serve fails, even after pip installing mkdocs -- it apparently doesn't auto-install extensions

[netlify[bot]]

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