Move some delegate docs into the new docs tree (#1082)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1082

Move `delegates.md` and `delegates_and_dependencies.md` into the new docs tree, fixing up their dependencies and adding them to the sidebar.

Reviewed By: mergennachin

Differential Revision: D50611724

fbshipit-source-id: 1cb16fe6035f4fc43fefceb2136f2d06e9ab7987