Document types of dependencies scanned by Dependency Scanning

Users of GitLab Dependency Scanning don't know if development dependencies, test dependencies, or other types of dependencies are being scanned.

The following table captures which types of dependency are scanned by the Gemnasium analyzers:

Analyzer	Package manager	What is scanned	Everything?	Issue
gemnasium	npm	default and dev dependencies	✅
gemnasium	yarn	default and dev dependencies	✅
gemnasium	Bundler	any dependency group	✅
gemnasium	PHP Composer	default dependencies only	❌	#343041 (closed)
gemnasium-maven	Maven	all dependency scopes	✅
gemnasium-maven	Gradle	all dependency configurations	✅
gemnasium-maven	Sbt	`compile` configuration only	❌	#343043
gemnasium-python	pip	N/A	✅
gemnasium-python	Setuptools	N/A	✅
gemnasium-python	Pipenv	default and develop dependencies	✅

Cover the dependency types being scanned in Dependency Scanning user documentation.

groupcomposition analysis backend engineers, or anyone who's familiar enough with Dependency Scanning and package managers.