docs : add "Quick start" section for new users

[ngxson]

QoL change for new users, add a "Quick start" section on top, so they don't have to scroll through the page to get to the right place.

Another QoL change, I added a table to tell which platform uses which package manager:

[slaren]

I have never heard of flox before, and from what I can tell it is just using the nix package. Generally I am not sure about promoting packages that we have no control over.

[mofosyne]

@slaren have not heard of it before, but it looks to already be present under section "Building the project" in "README.md" and "Flox" in docs/install.md . So I suspect he was just doing a carry over of that section.

It's worth however considering if we should delete this term from the repo in a separate pull request.

[slaren]

Ok, but still, there is a difference between listing a package as the recommended way to use llama.cpp, and as a note at the bottom of install.md. I think it is ok to give a shout out to other projects, but recommending them should require a different standard. In this case though, I am not sure that we need to mention flox at all, since it seems to be only a different way to install the nix package.

[ngxson]

I don't know about flox either, I think it was added before I joined the project as contributor, so I have no visibility about it.

If any nix packages is usable in flox (which seems to be the case), then it will be a common knowledge for flox users to just use whatever nix package that they want. Therefore, I think it's not useful to mention flox here.