Document bespoke library strategy.

[elliotcmorris]

What

Come up with, and document the strategy/recommendations for the use cases where DCC authors will want to extend their trait/spec environment with product/company specific traits.

We'll need to decide on exactly how we expect custom traits to be distributed, ie via .yml or via distributing headers directly. Relates to #61

We'd also be well placed deciding on the "tone" of how we talk about custom traits. Are they something to be avoided if possible? We wanna avoid DCC authors only considering their own product at the expense of the ecosystem.

Acceptance Criteria

• Provide examples/guidance on when custom traits are appropriate
• Provide guidance for when a trait should be considered for "promotion" to mediacreation
• Document that integrations must not depend on knowledge of the product specific trait, and should be functional without them.
• Put this in the Mediacreation README

### Tasks
- [ ] https://github.com/OpenAssetIO/OpenAssetIO-TraitGen/issues/30
- [ ] https://github.com/OpenAssetIO/OpenAssetIO-TraitGen/issues/76

[feltech]

I've added pre-existing tasks that are related to / duplicate this and changed this to an epic.