org: convert all images to svg

Suggested in #299.

* doc/org/autfilt.org, doc/org/concepts.org, doc/org/dstar2tgba.org,
doc/org/genaut.org, doc/org/hierarchy.org, doc/org/hoa.org,
doc/org/ltl2tgba.org, doc/org/ltl2tgta.org, doc/org/ltlcross.org,
doc/org/oaut.org, doc/org/randaut.org, doc/org/satmin.org,
doc/org/tut11.org, doc/org/tut23.org, doc/org/tut24.org,
doc/org/tut30.org, doc/org/tut31.org, doc/org/tut50.org,
doc/org/tut51.org: Adjust all dot outputs to produce svg.
* doc/org/arch.tex, doc/org/hierarchy.tex, doc/org/satmin.tex: Adjust
to produce a pdf with 12pt text.
* doc/Makefile.am: Adjust the generation of arch.svg, hierarchy.svg,
and satmin.svg: From above.
* doc/org/.dir-locals.el.in, doc/org/init.el.in: Adjust dot arguments
to produce svg with 12pt text (the default was 14pt).
* doc/org/spot.css: Use Lato as the main font for consistency with
automata.
* HACKING: pdf2svg is now required to build the doc.

doc/org/arch.tex

@@ -1,4 +1,4 @@
-\documentclass[convert={size=640}]{standalone}
+\documentclass{standalone}
 \usepackage{tikz}
 \usetikzlibrary{arrows}
 \usetikzlibrary{arrows.meta}
@@ -9,6 +9,7 @@
 
 \begin{document}
 
+\scalebox{1.2}{
 \begin{tikzpicture}
   \tikzset{node distance=2mm,
            basicbox/.style={minimum width=#1,minimum height=8mm},
@@ -81,7 +82,7 @@
 ($(libspot.south east)+(1mm,-1mm)$) --
 ($(libspot.south west)+(-1mm,-1mm)$) -- cycle;
 \end{pgfonlayer}
-\end{tikzpicture}
+\end{tikzpicture}}
 \end{document}
 %%% Local Variables:
 %%% mode: latex