Wrong repo paths mentioned in getting started doc for latest and greatest `fyne` and `fyne_demo`

[bobhy]

Description

It looks like both the fyne tool and fyne_demo app were recently copied out of the fyne repo.
The code remaining in the old repo now prints a deprecation message.

The getting started doc, however, still references the old paths. So the new user encounters awkward deprecation messages when forming their first impressions of the (technically impressive) toolkit.

$ go install go install fyne.io/fyne/v2/cmd/fyne@latest
$ fyne
NOTE: This tool is deprecated and has migrated to fyne.io/tools/cmd/fyne. The new tool can be installed by running the following command:
	go install fyne.io/tools/cmd/fyne@latest

I've submitted #49 to track the moved repo paths.

However, the demo repo path has a couple of problems:

1. Rename repo demo to fyne_demo so the module (and executable) name can match the GUI window name.
2. Fix fyne.io to redirect fyne.io/fyne_demo to github.com/fyne-io/fyne_demo (I had to document the github path, which is a wart.