[doc] move doc/assumeutxo.md to doc/design

Summary: Another doc file about how to use assumeutxo will be added as doc/assumeutxo.md (core#29553), so we need to move the first one (done in core#24352)

Test Plan: NA

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18125