Update Doxygen config for generating online API docs

Enabled implicit brief descriptions (`JAVADOC_AUTOBRIEF`), and markdown
support (`MARKDOWN_SUPPORT`) for doc blocks. This means that we no
longer need to explicitly inclufe a `@brief` tag in the doc block, the
first full sentence will be regarded as the brief documentation if it's
not explicitly given. Also we can use Markdown formatting in the
comments, which is a bit nicer than the native Doxygen tags.

I also disabled the Doxygen_phpvarfilter, but leave it commented out. It
should not be needed anymore unless somebody is using an ancient version
of doxygen. (Don't do that!)

I also changed the heading a bit, removed "The" from "The Hubzilla", and
added a tagline. Feel free to revise to whatever conforms to the project
norms.

util/Doxyfile

@@ -1,6 +1,7 @@
 INPUT = README.md index.php boot.php include/ install/ util/ view/ Zotlabs/
 RECURSIVE = YES
-PROJECT_NAME = "The Hubzilla"
+PROJECT_NAME = "Hubzilla"
+PROJECT_BRIEF = "A powerful, privacy oriented fediverse CMS."
 PROJECT_LOGO = images/hz-64.png
 IMAGE_PATH = images/
 EXCLUDE = .htconfig.php library/ doc/ store/ vendor/ .git/ util/zotsh/easywebdav/ util/generate-hooks-index/
@@ -36,4 +37,6 @@ COLLABORATION_GRAPH = NO
 # fix @var (https://bugzilla.gnome.org/show_bug.cgi?id=626105)
 # Should be obsolete with doxygen >= 1.8.15
 #INPUT_FILTER = "sed -e 's/@var\s/@see /'"
-INPUT_FILTER = "php util/Doxygen_phpvarfilter.php"
+#INPUT_FILTER = "php util/Doxygen_phpvarfilter.php"
+JAVADOC_AUTOBRIEF = YES
+MARKDOWN_SUPPORT = YES