doc/sphinx: Further configure doxygen

Turn warnings into errors so we can spot them immediately.

While at it: no need to generate class graphs as we're not using it and
make doxygen run quieter.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

doc/sphinx/doxygen.ini.in

@@ -724,7 +724,7 @@ CITE_BIB_FILES         =
 # messages are off.
 # The default value is: NO.
 
-QUIET                  = NO
+QUIET                  = YES
 
 # The WARNINGS tag can be used to turn on/off the warning messages that are
 # generated to standard error (stderr) by doxygen. If WARNINGS is set to YES
@@ -762,7 +762,7 @@ WARN_NO_PARAMDOC       = NO
 # a warning is encountered.
 # The default value is: NO.
 
-WARN_AS_ERROR          = NO
+WARN_AS_ERROR          = YES
 
 # The WARN_FORMAT tag determines the format of the warning messages that doxygen
 # can produce. The string should contain the $file, $line, and $text tags, which
@@ -2214,7 +2214,7 @@ HIDE_UNDOC_RELATIONS   = YES
 # set to NO
 # The default value is: YES.
 
-HAVE_DOT               = YES
+HAVE_DOT               = NO
 
 # The DOT_NUM_THREADS specifies the number of dot invocations doxygen is allowed
 # to run in parallel. When set to 0 doxygen will base this on the number of