Currently, any warning during the documentation compilation causes an exception which stops the compilation.

This prevents adding certain cross links in the reference
manual. Indeed an undefined warning is issued during the first pass of
the compilation whenever some document A cross links to some document
B while A happens to be compiled before document B.

With this ticket, warnings during the first pass of compilation don't
trigger an exception any more. This ticket also adds a cross link from
sage.dynamics to sage.combinat to illustrate and test the issue.

CC: @hivert @jhpalmieri @sagetrac-sage-combinat @vbraun

Component: documentation

Author: Florent Hivert, Nicolas M. Thiéry

Branch: fb43b19

Reviewer: John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/17189