Preview: https://docs-preview.pytorch.org/pytorch/pytorch/157750/export.html
Changes:
* Rename draft_export.md -> export.draft_export.md for consistency.
* Removed non-strict section in export, instead pointed to programming model doc.
* Extended "Expressing Dynamism" section to include Dim hints, ShapeCollection, and AdditionalInputs.
* Removed Specialization section in favor of programming model doc
* Added pt2 archive doc
* Cleaned up sidebar
Pull Request resolved: https://github.com/pytorch/pytorch/pull/157750
Approved by: https://github.com/pianpwk
