[DOC] Set up Sphinx doc build

[svekars]

• Add the requirements.txt and Makefile for the docs: - Enable sphinx 5.0, sphinx-gallery, myst-parser, sphinx-design
• Create the initial toctree and placeholder files under source/
• Add custom directive for the customcards on the landing page
• Configure conf.py to use sphinx-gallery, myst-parse, custom directives, etc.
• Add custom.css for the sphinx-design cards
• Add template_tutorial.py to demonstrate how to create tutorials

A quick preview:
https://github.com/pytorch/executorch/assets/5317992/12408b00-43e1-4447-878f-a6a43409ea63

[dbort]

This is a great foundation for us to build on! Having the TBA docs carved out is especially helpful, since it shows us what we need to work on.

Looks good overall, but note that we don't support PR merges yet, so you'll need to import this into an internal diff to land.

[dbort]

The executorch repo currently has a checked-in directory named build, so we can't ignore it here, and we can't use build as the place for temporary artifacts. Would it be possible to use a name like docs-out? Maybe even put sphinxbuild_{py,cpp} under that top-level output directory to keep things cleaner at the root?

[dbort]

These URLs refer to _static/images/..., but the .svg files live under _static/img/...

Though I see that index.rst does use the /img/ URLs.

[dbort]

I found some dynamo docs at https://pytorch.org/docs/stable/dynamo/index.html, but nothing specific to torch.export. We should provide links to the readers so they don't need to hunt them down, but I recognize that this is a first-pass placeholder doc.

[dbort]

Suggested change

	and how yo can use them in your projects.
	and how you can use them in your projects.

[dbort]

Suggested change

	Getting Starrted Tutorials
	Getting Started Tutorials