docs: Improve adding components documentation

This merge request introduces several improvements to the "Add new GitLab UI component" documentation:

• It breaks the list of steps into several sections.
• It puts more emphasis on using yarn generate:component.
• It adds an image to clarify where to find the update screenshots job.
• It adds sections about Pajamas and wrapping BootstrapVue components.

The goal is solving the issues described in #622 (closed)