Fix out of date documentation & remove friction points (#78682)
Fixes various friction points with the documentation for onboarding new users and remove instructions that were no longer valid
Changes include:
- Listing prerequisites earlier, so that devs can ensure they're met before encountering error messages
- Removing linter invocations that are no longer valid
- Modifying instructions to install mkl packages to only apply to x86 based CPUs
[skip ci]
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78682
Approved by: https://github.com/seemethere, https://github.com/janeyx99, https://github.com/malfet
