Update FE guides with dependency/devDependency distinction

Makes clear the distinction between dependencies and devDependencies within our package.json for GitLab's purposes. These boundaries are a little fuzzy because different projects use these categories for different purposes. Given that GitLab is not a node project and there are no dependencies required when running GitLab in production, we've opted to consider our "production" assets ones which are required to compile our webpack bundles. This way the omnibus project and users who compile GitLab from source will be able to install a subset of our dependencies via yarn install --production and still be able to complete the build process.

This distinction is not new (if you accidentally added a dependency required by omnibus to devDependencies, the gitlab:assets:compile CI job would fail), but it has never been canonized in our FE docs.

Relevant discussion: gitlab-org/gitlab-ce!19231