Add optional table of contents to posts

As some of our pages include a lot of content, structured through the
use of headings, it makes sense to include an auto-generated ToC in our
template.

This ToC is only generated for pages including the `toc` boolean
parameter with value `true` in their source header, so it can be enabled
only when really useful.

It only includes elements from heading levels 2 and 3 by default, while
most of our documents are structured by using levels 1 and 2, so this
commit also includes changes to the config file to reflect that.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>