docs: Add quickstart

[ym-han]

I made a separate quickstart doc and linked to it from the README, to make the README less cluttered -- that's how https://github.com/Eventual-Inc/Daft/blob/main/README.rst does it too. But I could also inline it into the README if that's what is preferred.

The quickstart doesn't get you to run a quick Mujoco simulation, but maybe it should. Then again, it's also important for a quickstart to be super concise.

The README needs more cleaning up -- I would personally go for a structure like Daft's -- but that probably shouldn't be part of this PR.

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[spomichter]

this wouldnt work im pretty sure since dimos isnt in PyPi public distro yet

[ym-han]

yes, the install instructions are aspirational / placeholders -- there's a comment to that effect. But maybe I should make it clearer it's TBD

[ym-han]

I've updated it to make it more explicit that the installation instructions part is just a placeholder. Hopefully after #731 lands and after release, it really will be that easy to install dimos. (Actually after that PR lands, it might be possible to do pip install git+ssh://git@github.com/dimensionalOS/dimos.git even without adding it to PyPi).

(I could link to the current installation instructions, but I really do think the installation needs to be made simpler for non-contributor users. The user-oriented quickstart needs to look quick.)