Add section and other markers to HTML help

[gaborcsardi]

E.g.

• anchors to the sections, so we can link to them, especially when we solve cross-page in-page linking,
• metadata to the links, to show the name of the originally linked topic, because after Link to files, not topics #1109 cross-package links will contain the file name, but not the topic.
• etc.

[hadley]

I think this is probably mostly no longer important in the age of pkgdown.