[doc] Add productivity note about utilizing the ramdisk for faster testing

Summary: Post review of D17965, it should be documented otherwise it will go unused during development which would be a shame.

Test Plan: Read and follow the instructions, then run ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17972