Diviner: mention how to flag Arcanist and Phorge as "safe" git repos

Summary:
This tip is useful if you are affected by the following error in git or similar:

    fatal: detected dubious ownership in repository at '/var/www/phorge'

Just follow the new tip from Diviner to fix at system level.

Ref T15282

Test Plan:
- see if you like the documentation tip
- try to execute the command, and verify that your computer does not explode
- eventually visit the /config/ page with and without that change (with and without a /etc/gitconfig)

Reviewers: O1 Blessed Committers, avivey, Cigaryno

Reviewed By: O1 Blessed Committers, avivey, Cigaryno

Subscribers: avivey, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15282

Differential Revision: https://we.phorge.it/D25236

src/docs/user/installation_guide.diviner

@@ -137,6 +137,12 @@ dependencies:
   somewhere/ $ git clone https://we.phorge.it/source/arcanist.git
   somewhere/ $ git clone https://we.phorge.it/source/phorge.git
 
+After cloning, flag them as safe (to avoid the error //"fatal:
+detected dubious ownership in repository at ..."//):
+
+  $ sudo git config --system --add safe.directory somewhere/arcanist
+  $ sudo git config --system --add safe.directory somewhere/phorge
+
 Next Steps
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