Allow doc to build with SAGE_PARI_CFG undefined #28341
Description
Follow up to #28242 which introduced SAGE_PARI_CFG in documentation build to identity whether or not pari is external.
This is inconvenient to distros and the behavior for external pari should be assumed if SAGE_PARI_CFG is undefined.
Depends on #28242
Component: distribution
Author: François Bissey
Branch/Commit: 27f17ff
Reviewer: Dima Pasechnik
Issue created by migration from https://trac.sagemath.org/ticket/28341