Update install instructions

[jeffdgr8]

The install instructions on the website and readme both describe using go get, which is deprecated for installing executables since Go 1.17.

go install doesn't support -u and -d options. And the resulting source location is different than described as well ($GOPATH/pkg/mod/github.com/magefile/mage@<version>).

When running either mage install or go run bootstrap.go from this source directory, I get the error:

Error: error creating generated mainfile: open mage_output_file.go: permission denied

[egor-denysenko]

I found out that installing mage with the following command:
go install github.com/magefile/mage@latest (Or pick the specific version that you want to install.)
and having the GOPATH and GOROOT properly evaluated it's working fine.
I could init a magefile config with mage-init instead of using go run bootstrap.go.
Keep in mind that i did this procedure with:
go version go version go1.21.5 linux/amd6

If it could help i can open a PR to update the docs and the readme

[egor-denysenko]

I have created the PR in order to update the docs here