Clarify backend terminology in API and adapter configuration

[jc-macdonald]

Context: follow-up from PR #75 review comments.

Problem

• Reviewer flagged potential overload/ambiguity of the term backend in adapter config and compile API discussions.
• Need clearer naming and documentation to distinguish array namespace selection from other engine/provider concepts.

Scope

• Propose and apply clearer terminology (for example, array_backend or equivalent) where it improves clarity.
• Document chosen term and migration/compatibility approach.
• Ensure config/docs remain aligned between op_system and flepimop2-op_system.

Acceptance criteria

• Terminology decision documented in code/docs.
• User-facing config/docs avoid ambiguous backend wording or clearly define it.
• Any rename/change is backward compatible or explicitly migration-noted.

Refs

• PR Make op_system callable JAX-native for diffrax/NUTS workflows #75
• Review comments in flepimop2-op_system/src/flepimop2/system/op_system/__init__.py around line ~69