doc: Add separate productivity notes document

Summary:

Many developers have their own tools and tricks to be more productive
during their cycles, so let's document the best ones so that everyone
can benefit from them.

I adapted the paragraph on how to compile with features disabled for cmake, and dropped the make -j paragraph as ninja seems to use all CPU cores already.

This is a backport of Core PR15348

Test Plan: Proof-reading.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8435