doc: update syntax-highlighter scalability info

Created by: michaellzc

Follow up on https://sourcegraph.slack.com/archives/CHXHX7XAS/p1646783148867999

syntax-highlighter is a stateless service and has no external dependencies (e.g. pg), so it should be horizontally scaleable.

Test plan

sg run docsite 

Go to http://localhost:5080/dev/background-information/architecture#diagram

syntech server should be surrounded by a box instead of a rectangle.

Should we experiment it on dogfod? https://github.com/sourcegraph/deploy-sourcegraph-dogfood-k8s/pull/4008 merged dogfood syntaxhighliting seems fine to me container logs look fine too