Replace header attribute contexts with sections

[Witiko]

As described in Section 2.4 of our article in TUGboat 43:1, header attribute contexts surround entire sections of content, which makes it difficult to react to attributes by inserting code after a header (rather than after a section) and makes it impossible to react to arbitrary sections, which may or may not have headings with attributes.

This issue proposes to create separate renderers for sections and make the header attribute contexts span just headings. We can introduce the section renderers early but changing the semantics of the header attribute contexts is a breaking change that should be postponed until version 3.0.0.