[DOC] Update build documentation and contributing to use cmake and ninja

Summary:
Includes some fixes and refreshes here and there.
All these instructions have been tested working on the following
platforms:

• FreeBSD 12.0
• MacOS Catalina 10.15.1
• Ubuntu Bionic 18.04 (WSL)
• Debian Buster (10)
• Fedora 31

Test Plan: Read the documents, follow the instructions and check it works !

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4748