docs: add 'how to install' to argo-cli

[asfaltboy]

Context

I was missing this bit of information when I got to this page. Also includes a reference of using homebrew.

Testing

Checked the markdown renders in github.

Going to try and rely on CI to make docs, if that doesn't happen will update with testing strategy.

Update: need to address the failing "spelling check"

[terrytangyuan]

I am not sure if the changes are necessary. The installation instruction is here: https://github.com/argoproj/argo-workflows/blob/83fa63794a958881a2cd24182a4f101889d68f15/docs/installation.md

It refers to more specific links to install from releases.

[asfaltboy]

The installation instruction is here

I know this is pretty basic. My goal is to improve the discoverability. I wanted to install the CLI tool, and searching through the docs brought me to this page, not the release page. Note that the release page doesn't mention Argo CLI, and I was less likely to follow to that page (I didn't need to install argo server or controller)

Would it better if the page should instead instruct users to refer to the releases.md page for up to date instructions (to avoid maintaining two methods of downloading)? Happy to change it if this makes sense, could also add a note there about what's available on the releases page.

On a related note, do you think the release notes should also mention homebrew, or you feel it's better to instruct users to download the pre-compiled binary as we do today?

[agilgur5]

Recreated in #11751

Would it better if the page should instead instruct users to refer to the releases.md page for up to date instructions (to avoid maintaining two methods of downloading)? Happy to change it if this makes sense, could also add a note there about what's available on the releases page.

I suggested adding a link to the GH Releases page in that PR, which was accepted