Add xindy to pdf-docs prereqs

xindy[0] is an index processor used by LaTeX. Instead of disabling it in
all projects, let's add it to prereqs and see how it goes. See also [1].

[0] https://ctan.org/pkg/xindy?lang=en
[1] https://review.opendev.org/684451

Change-Id: Iaef8a36dc24b0aacb11454d112044c80613d5263

roles/prepare-build-pdf-docs/tasks/main.yaml

@@ -12,6 +12,7 @@
       - texlive-fonts-recommended
       - fonts-liberation
       - inkscape
+      - xindy
       # Required by sphinxcontrib-svg2pdfconverter to handle SVG images
       - librsvg2-bin
     state: present