Add complete example/docs on adding traits and specifications

[foundrytom]

What

Properly document the process of adding custom traits + specifications to a Host, so we have good examples of the ideas, process and resulting YAML, including cross-package references (eg. back to MediaCreation)

ACs

• Example of adding host-specific specifications following on from a link to the main API docs.
  • Pointers to API orientation docs
  • Worked YAML authoring example with cross-package trait references
  • Basic CLI generation
  • Basic usage example (python)
  • Calls out best practices and "dont do this" in appropriate sections.
  • Format TBD in https://github.com/OpenAssetIO/OpenAssetIO/issues/946

Out of scope

• CI/build system integration
• In-depth usage examples
• C++

[elliotcmorris]

Relates to / may be prerequisite of : https://github.com/OpenAssetIO/OpenAssetIO-MediaCreation/issues/92