Trac #31397: Work around LaTeX bug for building pdf documentation

Very recent versions of LaTeX won't build the PDF documentation for
`en/reference/stats/`, instead yielding an error:
{{{
Chapter 8.
! TeX capacity exceeded, sorry [input stack size=5000].
\non@alpherr #1->\@latex@error {\string #1
                                          allowed only in math
mode}\@ehd
l.2922 ...Samplers for \protect\(\ZZ[x]\protect\)}
}}}
We should find a workaround.

(Split off from #31387 — some more discussion there.)

LaTeX bug is fixed at latex3/latex2e#503.

URL: https://trac.sagemath.org/31397
Reported by: jhpalmieri
Ticket author(s): John Palmieri
Reviewer(s): Dave Morris

Author: Release Manager

src/sage/docs/conf.py

@@ -507,6 +507,14 @@ def set_intersphinx_mappings(app, config):
 
 \let\textLaTeX\LaTeX
 \AtBeginDocument{\renewcommand*{\LaTeX}{\hbox{\textLaTeX}}}
+
+% Workaround for a LaTeX bug -- see trac #31397 and
+% https://tex.stackexchange.com/questions/583391/mactex-2020-error-with-report-hyperref-mathbf-in-chapter.
+\makeatletter
+\pdfstringdefDisableCommands{%
+  \let\mathbf\@firstofone
+}
+\makeatother
 """
 
 # Documents to append as an appendix to all manuals.