asciidoctor: Fix user-manual to be built by asciidoctor

The `user-manual.txt` ist designed as a `book` but the `Makefile` wants to
build it as an `article`. This seems to be a problem when building the
documentation with `asciidoctor`. Furthermore the parts *Git Glossary*
and *Apendix B* had no subsections which is not allowed when building with
`asciidoctor`. So lets add a *dummy* section.

Signed-off-by: nalla <[email protected]>

Author: nalla

Documentation/Makefile

@@ -335,7 +335,7 @@ manpage-base-url.xsl: manpage-base-url.xsl.in
 
 user-manual.xml: user-manual.txt user-manual.conf
 	$(QUIET_ASCIIDOC)$(RM) $@+ $@ && \
-	$(TXT_TO_XML) -d article -o $@+ $< && \
+	$(TXT_TO_XML) -d book -o $@+ $< && \
 	mv $@+ $@
 
 technical/api-index.txt: technical/api-index-skel.txt \

Documentation/user-manual.txt

@@ -3424,7 +3424,7 @@ just missing one particular blob version.
 
 [[the-index]]
 The index
------------
+---------
 
 The index is a binary file (generally kept in `.git/index`) containing a
 sorted list of path names, each with permissions and the SHA-1 of a blob
@@ -4366,6 +4366,10 @@ itself!
 Git Glossary
 ============
 
+[[git-explained]]
+Git explained
+-------------
+
 include::glossary-content.txt[]
 
 [[git-quick-start]]
@@ -4607,6 +4611,10 @@ $ git gc
 Appendix B: Notes and todo list for this manual
 ===============================================
 
+[[todo-list]]
+Todo list
+---------
+
 This is a work in progress.
 
 The basic requirements: