Update Orch Docs from User's Perspective

[amessbee]

As per discussions with Dean and Rowland, our orchestration docs do a reasonable job of showcasing features of Orchestration. However, there is need to update them to provide a quick ramp for otherwise experienced developers to use orchestration API in their new or mature cross-chain use cases.

• Update Orchestration landing page, to showcase how easy it is to use Orch API to solve several use cases with code examples.
• Add (may be a separate page of) details of distinction between transactional vs portfolio use cases and how Orchestration helps with examples.
• Update Key concepts page on similar lines as above.

[dckc]

should this be "from Dev's perspective"?