Change landing page to display TOC

[banchan86]

bonsai-rx/prefect#39 reminded me of this issue with the core docs website that I have been meaning to fix. This PR changes the landing page to the "Install Bonsai" page, which I believe enhances accessibility of the core docs for the same reasons listed in the linked issue.

I tested this out, and it doesn't break the existing download link on bonsai-rx.org (both index.html and articles\installation.html are valid).

One controversial issue is that this PR removes the existing "Quick Start" guide that used to be in index.md. I thought about saving it as a separate article to include in the "Getting Started" section, but I believe it is somewhat redundant for two reasons:

• The "Install Now" button on bonsai-rx.org already bypasses the "Quick Start" by linking directly to the installation page. Hence, new users who download the software already do not encounter this page.
• The content on the "Quick Start" guide is duplicated by the existing articles.

I am open to saving the "Quick Start" article, but it means we should change the name of the "Getting Started" section to something like "Basics" or "Essentials" (the docfx website uses this). Alternatively, we can merge some of the content into the "Install Bonsai" page.

Also removed a few extra lines that I noticed.