Add a separate customization guide. Move documentation of the custom and
local sources there. Also, cover the new NodeFeatureRules custom
resource and the new expression-based label rule format.
This patch also simplifies the "Feature labels" page, describing
built-in labels. Reformat the tables describing feature labels.
Make table of contents in the pages cleaner and more readable by
dropping the main heading (H1 level) from TOCs. This was the original
intention with the usage of "no_toc" kramdown magic, which was broken,
however. The kramdown class magic needs to be specified on the line
immediately following the headinds, otherwise it has no effect. We need
to disable MD022 rule of mdlint as it does not understand this magic.
