[docs] initial QT documentation, move Qt Creator instructions

Summary:
Backport of core PR11761
https://github.com/bitcoin/bitcoin/pull/11761/files

Test Plan: Read it, test it.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3910