docs: Set LaTeX engine to XeLaTeX for PDF generation.

Also added to suggested packages list for PDF build.

See comment in Sphinx project for (some) details:
https://github.com/sphinx-doc/sphinx/pull/5693#pullrequestreview-180444650

docs/README.md

@@ -47,7 +47,7 @@ This can be achieved with:
 
     make latexpdf
 
-but require rather complete install of LaTeX with various extensions. On
-Debian/Ubuntu, try (500MB+ download):
+but requires a rather complete install of LaTeX with various extensions. On
+Debian/Ubuntu, try (1GB+ download):
 
-    apt-get install texlive-latex-recommended texlive-latex-extra
+    apt install texlive-latex-recommended texlive-latex-extra texlive-xetex texlive-fonts-extra cm-super xindy

docs/conf.py

@@ -259,6 +259,8 @@ latex_documents = [
 # If false, no module index is generated.
 #latex_domain_indices = True
 
+# Enable better Unicode support so that `make latexpdf` doesn't fail
+latex_engine = "xelatex"
 
 # -- Options for manual page output ---------------------------------------