[DOC] Fix out of order sections in the developer notes

Summary:
The Valgrind and Lcov section have been landed in the wrong order,
leading to a section ordering reversal when compared to core document.
Reset to the correct ordering make further backport much easier.

Test Plan: Read the doc.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3961