Edit installation.md

[LarryUllman]

Notes:

• change title to "Install"
• page doesn't discuss configuration so I removed that text
• DB content goes on the self-hosted page (makes this page simpler and encourages "just use Cloud")
• removed esoteric content; can be placed elsewhere if it's necessary
• make the TOC cleaner and better targeted
• point to quickstart after this instead of the tutorial

(Possible) TODOs

• rename "Contributing" to "Contribute" and then update this page
• rename this file and slug to "install"
• add necessary content back into the self-hosted page