Add index pages to official sections

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