chore: docs and cleanup (rm Custom Inst)

[rakita]

Few minor changes and docs added to Interpreter Traits.

Output field removed from InstructionProvider.
DummyHost added.
CustomInstruction removed as not used.
buffer_mut renamed to set_buffer

[codspeed-hq]

CodSpeed Performance Report

Merging #2151 will improve performances by ×2.1

_{Comparing rakita/docs_cleanup (f890404) with main (435e630)}

Summary

⚡ 2 improvements
✅ 6 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	analysis	228.2 µs	178.1 µs	+28.16%
⚡	transfer	97.1 µs	46.5 µs	×2.1