Re-write export tutorial / export getting started

[GregoryComer]

📚 The doc issue

Currently, the Getting Started doc flow points users to the Exporting to ExecuTorch Tutorial. While this page has a lot of key information, it is likely too low level for many users. We need to split it out into a true "Getting Started" and an advanced export / export reference page. The getting started doc should provide a concise description of how to export + lower for the most common use cases without getting too far into the weeds on IR, operator sets, and advanced use cases.

Ideally, this doc should cover:

• End to end steps for exporting and lowering a model.
• Specifying dynamic inputs (this needs to be in the getting started guide, as it's a very common trap for users).
• Sanity checking the model using pybindings.

Suggest a potential alternative/fix

No response

cc @mergennachin @byjlw

[mergennachin]

Yeah, this is good!

[digantdesai]

Can we make sure the tutorial is in the CI?