Improve dox header implementation

The dox header contains information about a page, and how it should be published. The information takes the form of a page ID and source directives. Right now, the directives are simple flags that modify dox publishing behavior.

More directives have been added, exposing the need for a better, scalable solution of a dox header. Some things to keep in mind:

- solution should be scalable, allow for more directives to be cleanly added
- directives may not be boolean flags, allow for directives of different values

Start by tracing how the dox header is parsed and how the page ID and source directives reach publishing behavior. Compare the current boolean-flag handling with the need for directives carrying other values. Done means the header design can accept additional directives cleanly without being limited to boolean flags.

Written by the indexing model from the issue text.