Add a basic description to the index page for each of the "official"
sections (as laid out inthe websites README.md). This provides visitors to
the site with some guidance as to what to expect to find in each section,
somewhere to point out to related polices (such as the release schedule and
flow from the releases section) and a way to add aliases (such as the
missing alias to Guides for the "Guidelines" section that was found on the
old wiki).

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>