Update Go install instructions

[GWevroy]

Hi,
Hopefully I am making this request in the right place. Recently a colleague and I were struggling to get Periph.io installed. We were using the "go get" command but as you are aware, how this works changed slightly as of version 1.17.x whereby it also modifies project mod files. Hence the current instructions will only work if you have an existing go project under development, and you are in that directory (and you are aware and happy with the fact that it is going to modify your mod files)

After some discussion on Slack, we found that the ideal means of installing the entire periph.io library is to use the go "install" method as follows:
go install periph.io/x/cmd/...@latest

this will install the latest version. As this was quite a runaround for us, I'm sure other people will be having the same issue. Having some up to date instructions will certainly help a developer (or newbie) to get started with a lot less frustration.

Thanks for a great IO library!

[maruel]

Hi! Can you make a PR? You can do it in the web by opening https://github.com/periph/website/blob/main/site/content/_index.md then pressing dot "." on your keyboard.