contributor docs: clarify when to install manually vs. using Docker

[kjhollen]

Increasing Access

This documentation amendment will make the installation guide more accessible for people with a moderate amount of developer experience, but who are new to Docker.

Feature enhancement details

In the Development Installation section of the contributor docs, there are Manual Installation and Docker Installation, but doesn't explain why you might choose one or the other. I'm comfortable with front end development and have a vague idea of what Docker does, but I haven't used Docker before. I might appreciate a sentence or two explaining when/why I would choose one mode of installation over another.