Update docs- mostly adding quickstart

[connor-french]

Summary

This PR fleshes out the README with installation details, rearranges the documentation front page, adds a quickstart section, and fixes a few things that came up when writing the documentation. Notably, I added a simulation module so users don't have to import msprime separately. It doesn't add functionality, just creates a couple spaceprime functions.

Related Issues

Fixes #8 #7 #28

Changes Made

• Fleshed out the README
• Added a Quickstart
• Added a simulation module, which thinly wraps the msprime sim_ancestry and sim_mutations functions
• Fixed a plotting issue

Motivation and Context

Adds more detailed documentation and fixed a few issues that came up while working on the documentation.

How Has This Been Tested?

No tests for the documentation. Just rendered the docs and made sure they worked.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules