Add instructions for how to install Chocolatey offline from a local nupkg

## Is Your Feature Request Related To A Problem? Please describe.

There have been users that want to install Chocolatey from a local, predownloaded `.nupkg`. There are not currently instructions on how to do that.

## Describe The Solution. Why is it needed?

Add install instructions for how to install Chocolatey from a local nupkg. It may be good to also create a guide for how to install both Chocolatey, and packages from predownloaded nupkgs, for users that want an offline setup without requiring a full on nexus or similar repository.

## Additional Context.

There is a PR that updates the `install.ps1` script to work with local `.nupkg` files. This documentaion would require that PR to be merged first, because the instructions would presumably use the updated `install.ps1` script.

## Related Issues

(From @corbob)
https://gitlab.com/chocolatey/community-infrastructure/community.chocolatey.org/-/merge_requests/1129