add incfig documentation

doc/wrappers/incfig/.latexmkrc

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+$makeindex = "makeindex -s gind.ist %O -o %D %S";
+$clean_ext .= ' glo';
+$clean_ext .= ' hd';
+$clean_ext .= ' ins';

doc/wrappers/incfig/Makefile

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+doc:
+	latexmk
+
+full:
+	latexmk -gg
+
+clean:
+	latexmk -C

doc/wrappers/incfig/incfig.tex

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+\documentclass[full]{l3doc}
+
+\title{The \pkg{mkessler-incfig} package}
+\author{Maximilian Ke\ss ler}
+
+\begin{document}
+
+\maketitle
+
+\begin{documentation}
+
+\begin{abstract}
+  This is some small wrapper around incfigures,
+  based on
+  \href{https://castel.dev/post/lecture-notes-2/}
+  {https://castel.dev/\allowbreak post/\allowbreak lecture-notes-2/}
+\end{abstract}
+
+\begin{function}{\incfig}
+  \begin{syntax}
+    \cs{incfig}{filename}
+  \end{syntax}
+
+  Inserts an incfigure.
+  The figures is assumed to be in the \texttt{figures/} folder.
+  The extension \texttt{pdf_tex} has \emph{not} to be given.
+\end{function}
+
+\end{documentation}
+
+\end{document}