New implementation of option reference/builddict generation.

Review Request #3314 — Created Jan. 7, 2016 and discarded

lahosken, zundel
- New implementation is in the docgen backend, not the internal
  backend, because repos might want to generate their own reference
  and builddict, that knows about their custom plugins.

- However our internal sitegen is modified to use this new docgen too,
  of course. To support this, two versions are generated: a standalone
  page (by default), and version suitable for embedding in the docs.

- The new pages have some minimal javascript, so they can expand/collapse
  sections of the doc.

- We only generate HTML pages. The old builddict generator also generated
  RST docs, but we didn't use them anywhere in practice.

CI passes:

Manually generated docs. See embedded versions here:

See standalone versions here:

Review request changed

Status: Discarded