Section CSS Classes for Headings

The class hX is rendered for the base class used. We need to render this as projectdoc-base-h1 to make clear that this is the base level and not the current level. The CSS style sheet needs to updated accordingly.

Since the class named changed, this is a breaking API change.

We also need to add the CSS class 'projectdoc-heading' to each heading rendered by the Section Macro