\documentclass{book}
\usepackage[spanish]{babel}
\title{Mi primer documento en \LaTeX}
\author{Celestino Rey López\thanks{\href{https://www.overleaf.com/learn/latex/Learn_LaTeX_in_30_minutes}{Aprende \LaTeX en 30 minutos}.}}
\date{}
\begin{document}
\part{La locura}
\chapter{First Chapter}

\section{Introduction}

This is the first section.

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neque pharetra sollicitudin.  Praesent imperdietmi nec ante. 
Donec ullamcorper, felis non sodales...

\section{Second Section}

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sollicitudin.  Praesent imperdiet mi necante...

\subsection{First Subsection}
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\section*{Unnumbered Section}
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\end{document}