Add required Mermaid JS styles for fixing exported SVG's to docs styles

Created by: ryan-blunden

Correct rendering of diagrams such as that in https://docs.sourcegraph.com/dev/architecture are a result of adding additional styles to the SVG. Obviously doing this for each diagram is a code smell and the required re-usable styles should be put in the docs styles.

I've captured the (currently) required stle additions at https://gist.github.com/ryan-blunden/be704f11b504b4cfae69cae7382ed3c3 so I don't forget what they are.