Add information to setup a VM to do gitian build manually

Summary:
This ports the documentation from https://github.com/bitcoin-core/docs/blob/master/gitian-building.md and adapt them to fit Bitcoin ABC.

Link to sections of release-process.md are now dead as this docmentation was removed from Bitcoin ABC.

Test Plan:
I was able to:

• follow the instruction and create a Debian VM.
• run the manual gitian build on said VM.
• run the offline gitian build on that VM.

I was unable to:

• Verifiy the vagrant documentation
• Verify that gitian-build.py works as advertised
• Create a Fedora VM

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2293