Make page's titles more usable

Homepage meta title only display the site name as its title.
Content and section pages display both the page name and site name for
the meta title. Page name is displayed first so it is easier to identify
the correct tab when several tabs are opened.
Use per page meta-description

Instead of using the same meta-description in every page, use each page
own description (already defined on the front-matter).
Use parameterized site URl in social metadata

Instead of using a hardcoded site URL for Twitter and OpenGraph
metadata, use the {{ site.url }} parameter.