sysdbd.conf(5): Documented the LoadPlugin and PluginDir options.
doc: Build and install HTML documentation as well.

For now, build HTML variants of the manpages but without any index file or
similar.
.gitignore: Ignore asciidoc generated HTML output.
doc: (Re)generate and install the default asciidoc CSS stylesheet.
doc: Added documentation for all existing plugins.

Added one manpage for each plugin.