Building the docs target now generates the reference and leaves finished
documents in the build tree's docs directory: pdf/ holds the compiled
PDF, html/ holds the browsable reference when that format is enabled.
Doxygen and LaTeX work under docs/tmp/, which clean removes along with
the rest of the tree.
The reference has a front page and a structure. The markdown documents
in docs/ are part of the input, OVERVIEW.md becomes the landing page,
and the rest follow as chapters ahead of the namespace, class, and file
reference. The module contract, previously a plain comment block
invisible to Doxygen, is a page in its own right.
Struct and enum members throughout the headers carry documentation, on
their own lines above what they describe.