[doc] Add suggestion to use Phorge Arcanist to CONTRIBUTING file

Summary: Phorge version of Arcanist is actually maintained and works properly.

Test Plan: I followed these instructions on Ubuntu 22.04 and things are working well.

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14338