Consolidate gRPC get started guides under "Tutorials"

[qin-guan]

Help us make content visible

• Tell us what search terms you used and how you searched docs.

  • Clicked through docs on the sidebar.

• Tell us what docs you found that didn't address your concern.

  • It was a bit hard to find documentation (or rather more of a step-by-step guide) on adding gRPC to an existing application, hence I created this issue. It's not really an entire new doc, but hopefully shifting things around a bit so information is easier to find.

Describe the new topic

Consolidate "Tutorials" to:

• Get started
  • An overview of the options available for gRPC, links to topics below + maybe REST transcoding?
• Create a new service
  • This doc
• Add to an existing project
  • Parts of this doc, parts of this doc too

I'd be happy to contribute to this (: