[Docs] Auto-diff documentation overhaul (shader-slang#6202)

* AD: Docs Update

* More documentation

* More documentation

* More docs fixes

* Cleanup documentation

* More docs polish. Add docs for the [Differentiable] attributes

* Fixup code sections

* Fixup

* Address review comments

* regenerate documentation Table of Contents

* Update docs with more playground links

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Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>
Co-authored-by: Yong He <yonghe@outlook.com>

Author: 3 people