Add site description

@aacid content/_index.md corresponds to the home page, and config.yaml contains configs and params for the whole site. Since both of them didn't specify any description, the description we got for the URL https://manifesto.kde.org (i.e. the home page) was actually from Bootstrap. What @carlschwan suggested (adding a description in content/_index.md) is enough for the home page, but adding a description in config.yaml ensures that every page of the site has a description, even in the case when that page doesn't have a description of its own, and it's not a normal content page. The home page is only one of those pages that are not normal content pages.

The description added in this MR follows the description of http://agilemanifesto.org/.