Emphasize reference handling in the book in a dedicated box

Also name URL index homogeneously.

hyperlinks.tex

@@ -1,8 +1,9 @@
 \titlespacing*{\chapter}{0pt}{0pt}{40pt}
 \chapter*{Hyperlink URL index}
+\label{\detokenize{appendix:hyperlink-url-index}}
 \addcontentsline{toc}{chapter}{Hyperlink URL index}
-\fancyhead[LO]{\small Links}
-\fancyhead[RE]{\small Links}
+\fancyhead[LO]{\small Hyperlink URL index}
+\fancyhead[RE]{\small Hyperlink URL index}
 
 The following pages contain a list of URLs for all hyperlinks included in this
 book.  A referenced URL like \dlhbhref{D1}{this one}, can be identified by

parts/howtousethisbook.tex

@@ -27,7 +27,7 @@ \section{How to read this book}
 \label{\detokenize{intro/narrative:how-to-read-this-book}}
 \sphinxAtStartPar
 First of all: be excited. DataLad can help you to manage your digital data
-workflow in various ways, and in this book you will use many of them right
+workflows in various ways, and in this book you will use many of them right
 from the start.
 There are many topics you can explore, if you wish:
 local or collaborative workflows, reproducible analyses, data publishing, and so on.
@@ -40,12 +40,18 @@ \section{How to read this book}
 with each. While learning, it will be easy to
 \sphinxstylestrong{find use cases in your own work for the commands you come across}.
 
+\begin{importantnote}[before title={\thetcbcounter\ }, check odd page=true]{References to information inside and outside this book}
 \sphinxAtStartPar
-Throughout the book numerous \sphinxstyleemphasis{terms} for concepts and technical components
+Whenever related content \textit{inside} this book is referenced, the page number of the box, section, or chapter in question is given.
+
+Throughout the book, numerous \sphinxstyleemphasis{terms} for concepts and technical components
 are used. They are all defined in a {\hyperref[\detokenize{glossary:glossary}]{\sphinxcrossref{\DUrole{std,std-ref}{Glossary}}}} (\autopageref*{\detokenize{glossary:glossary}}), and are printed
 with a glossary icon, such as {\hyperref[\detokenize{glossary:term-Git}]{\sphinxtermref{\DUrole{xref,std,std-term}{Git}}}}, or {\hyperref[\detokenize{glossary:term-commit-message}]{\sphinxtermref{\DUrole{xref,std,std-term}{commit message}}}}.
-Links to external resources have a superscript that you can find in the “Links” collection at the end of the book.
-And internal links reference the page number of the box, section, or chapter in question.
+
+References to external resources have a superscript \dlhbhref{D1}{like this one}.
+The corresponding URL can be found in the {\hyperref[\detokenize{appendix:hyperlink-url-index}]{\sphinxcrossref{\DUrole{std,std-ref}{Hyperlink URL index}}}} at the end of the book
+(\autopageref*{\detokenize{appendix:hyperlink-url-index}}).
+\end{importantnote}
 
 \sphinxAtStartPar
 As this book aims to be a practical guide it includes as many hands\sphinxhyphen{}on examples