Documentation improvements

- [ ] Example of adding an inline parser (say, abbreviations)
- [ ] Example of modifying existing HTML output
- [ ] Example of creating a new output format, e.g. roff man. (In this case there are some complexities, because the effect of an 'emph' might be `\f[I]`, but it might be `\f[BI]` (if we're already in a boldface section). We can handle that by making the IsInline type for roff be a newtype embeddding State EmphState Builder or something like that. Then the effect can be state dependent. (Maybe we should consider changing all the implementations to be like this? It would remove some of the complexity for e.g. footnotes.)