Build out Contribute index with download instructions and descriptions of primary directories

[Clay-Mysten]

Move git clone command to Contribute in prep for removal from tutorial
Add link to Install docs for getting binaries

[Clay-Mysten]

Hi team,

Adam had the excellent idea of summarizing our primary directories in our Contribute page. I have taken a first step but would like your input. Feel free to commit changes directly or provide them in comments. And of course add missing reviewers. Thanks!

[awelc]

Hi team,

Adam had the excellent idea of summarizing our primary directories in our Contribute page. I have taken a first step but would like your input. Feel free to commit changes directly or provide them in comments. And of course add missing reviewers. Thanks!

Let me elaborate on this a bit more. The main motivation for this whole change is to balance two things out:

1. We don't necessarily want people that just want to play with Sui via it's CLI (sui, wallet, etc.) to be forced to clone the entire repo as it's not really useful much for this kind of audience
2. At the same time, in some docs we are referring to code in Sui's repo and ask people to "download" it somehow (e.g., Move code to be published for playing the TicTacToe game). In these cases it would be convenient to point people at some common location where we describe how to clone our repo.

The contribute section seems like a decent place to put repo cloning instructions as this file targets folks that actually need to inspect Sui's source code. Having bare-bones cloning instructions could be OK, but it just feels better to also include a very brief code structure blurb there as well.

[awelc]

Stamped, but since this was already something that @Clay-Mysten and I discussed, it would be good to get at least one more opinion on this from someone that is not biased.