Fix 'getting started' links in examples folder (#979)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/979

Some of them are referring to the old deprecated doc.
Some of them are referring to the md file by doing '../../'. Let's use the website link directly.

Reviewed By: shoumikhin, dbort

Differential Revision: D50309082

fbshipit-source-id: ecac525db2e14f21f6ab1c964ea5fce205b0b5f3