[Doc] Refine content for adding services to a mesh

In Adding services to a mesh:

• Flip the text in this section with the text in About adding projects to a service mesh
• Shorten task titles. Ex: use “member roll”
• Break apart “About adding projects to a mesh.” Use the descriptions for adding projects to a mesh as conceptual topics right before the respective tasks. This will group related chunks of information and will help create navigation aids.
• Add illustrations to help communicate the different approaches for our customers