HaikuBook: Apply layout customizations after default Doxygen stylesheet

Previously the 'book.css' file was applied as the sole stylesheet for the
Doxygen generated documentation. This stylesheet now applies it as a secondary
stylesheet to override Doxygen's defaults. The main advantage is that when
Doxygen adds new features, they will now be styled by default.

I did my best to clean up the book.css file, though it is not unlikely that
there still is a lot of duplicate CSS. For now though, the output looks mostly
the same, the code blocks no longer have an empty line between each of the lines
and the tooltip text - while not working - are no longer visible on the bottom
of the page.

Change-Id: Ia50915245ecd981843d0d8ec42e362b538187920
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3269
Reviewed-by: Adrien Destugues <[email protected]>

docs/user/Doxyfile

@@ -1195,7 +1195,7 @@ HTML_FOOTER            = footer.html
 # obsolete.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_STYLESHEET        = book.css
+HTML_STYLESHEET        =
 
 # The HTML_EXTRA_STYLESHEET tag can be used to specify additional user-defined
 # cascading style sheets that are included after the standard style sheets
@@ -1208,7 +1208,7 @@ HTML_STYLESHEET        = book.css
 # list). For an example see the documentation.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_EXTRA_STYLESHEET  =
+HTML_EXTRA_STYLESHEET  = book.css
 
 # The HTML_EXTRA_FILES tag can be used to specify one or more extra images or
 # other source files which should be copied to the HTML output directory. Note
@@ -1750,7 +1750,7 @@ COMPACT_LATEX          = NO
 # The default value is: a4.
 # This tag requires that the tag GENERATE_LATEX is set to YES.
 
-PAPER_TYPE             = a4wide
+PAPER_TYPE             = a4
 
 # The EXTRA_PACKAGES tag can be used to specify one or more LaTeX package names
 # that should be included in the LaTeX output. The package can be specified just