Add a CLI getting started page

[b5]

What feature or capability would you like?

A new page on our website: qri.io/cli that includes details for downloading qri CLI and QuickStart instructions

Do you have a proposed solution?

General details the page should include:

• should say qri CLI is available for mac, windows & linux
• users can install via curl: curl -fsSL https://qri.io/install.sh | sh
• they can also download from our GitHub releases page

Quickstart
Users should run the following to get started:
$ qri setup

$ qri registry signup --email YOUR_EMAIL_or, if you already have a qri.cloud account:_qri registry prove --email YOUR_EMAIL`

• add --help to any command for a detailed description of the command
• more info at qri.io/docs

Next link should be to docs

[feep]

From (private) Discord.

rico: this already exists b5 i mean, i'm all for promoting CLI to a direct subdomain as opposed to tucking it under docs, but maybe just make the page slicker/flashier, more welcoming, give it a hero & intro
b5: exactly
b5: I think we should just make the page flashier
b5: and have a shorter link