Declare the documentation's structure in a source file

src/documentation.cpp is the documentation loading source: it carries no
code and names each document that the generated reference contains, in
order, by page label. The labels are declared on each markdown file's
first heading. The build configuration lists the files as Doxygen inputs
and nothing more, so the structure of the documentation and the
mechanics of generating it are separate.

The hand-maintained document index at the end of OVERVIEW.md is gone,
along with the last references to markdown files by filename in the
public header. The document list exists in one place.

The docs target clears its output and scratch directories on every run.
Doxygen keeps output files it judges unchanged, so a run over a
populated scratch directory carried pages forward from earlier ones, and
removed sections survived in the generated HTML and PDF after their
source was edited.

docs/CONSUMERS.md

@@ -1,4 +1,4 @@
-# Consuming libdpm-core.so
+# Consuming libdpm-core.so {#consumers}
 
 Programs link libdpm-core.so as an ordinary shared library dependency — the same way they link any other library — to reach the package manager in-process: build systems, installers, image builders, system tooling, and foreign-language bindings all use the library the `dpm` binary is built on. A program holding a default context is working against system configuration, the system module path, the system tree, and system locking, the same environment the installed `dpm` binary sees.