doc/Doxyfile: Change to HTML generation, specify css

We now generate html and xml documentation!

The css is based on the default doxygen css with minor modifications to
improve readability (mainly setting the max width of the body to 800px).
diff --git a/doc/Doxyfile b/doc/Doxyfile
index 32fa4b8..0054887 100644
--- a/doc/Doxyfile
+++ b/doc/Doxyfile
@@ -38,7 +38,7 @@
 # could be handy for archiving the generated documentation or if some version
 # control system is used.
 
-PROJECT_NUMBER         = 1.3.22
+PROJECT_NUMBER         = 2.1.0-pre
 
 # Using the PROJECT_BRIEF tag one can provide an optional one line description
 # for a project that appears at the top of each page and should give viewer a
@@ -216,7 +216,7 @@
 # uses this value to replace tabs by spaces in code fragments.
 # Minimum value: 1, maximum value: 16, default value: 4.
 
-TAB_SIZE               = 3
+TAB_SIZE               = 4
 
 # This tag can be used to specify a number of aliases that act as commands in
 # the documentation. An alias has the form:
@@ -927,7 +927,7 @@
 # (index.html). This can be useful if you have a project on for instance GitHub
 # and want to reuse the introduction page also for the doxygen output.
 
-USE_MDFILE_AS_MAINPAGE =
+USE_MDFILE_AS_MAINPAGE = intro.txt
 
 #---------------------------------------------------------------------------
 # Configuration options related to source browsing
@@ -1048,7 +1048,7 @@
 # If the GENERATE_HTML tag is set to YES, doxygen will generate HTML output
 # The default value is: YES.
 
-GENERATE_HTML          = NO
+GENERATE_HTML          = YES
 
 # The HTML_OUTPUT tag is used to specify where the HTML docs will be put. If a
 # relative path is entered the value of OUTPUT_DIRECTORY will be put in front of
@@ -1105,7 +1105,7 @@
 # obsolete.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_STYLESHEET        =
+HTML_STYLESHEET        = docs.css
 
 # The HTML_EXTRA_STYLESHEET tag can be used to specify additional user-defined
 # cascading style sheets that are included after the standard style sheets
@@ -1592,7 +1592,7 @@
 # If the GENERATE_LATEX tag is set to YES, doxygen will generate LaTeX output.
 # The default value is: YES.
 
-GENERATE_LATEX         = YES
+GENERATE_LATEX         = NO
 
 # The LATEX_OUTPUT tag is used to specify where the LaTeX docs will be put. If a
 # relative path is entered the value of OUTPUT_DIRECTORY will be put in front of
@@ -1663,7 +1663,7 @@
 # to HTML_HEADER.
 # This tag requires that the tag GENERATE_LATEX is set to YES.
 
-LATEX_HEADER           = header.tex
+LATEX_HEADER           =
 
 # The LATEX_FOOTER tag can be used to specify a personal LaTeX footer for the
 # generated LaTeX document. The footer should contain everything after the last
@@ -1872,7 +1872,7 @@
 # captures the structure of the code including all documentation.
 # The default value is: NO.
 
-GENERATE_XML           = NO
+GENERATE_XML           = YES
 
 # The XML_OUTPUT tag is used to specify where the XML pages will be put. If a
 # relative path is entered the value of OUTPUT_DIRECTORY will be put in front of