Documentation generation produces consumable output in one command

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.

src/bundled-modules/info/include/commands.hpp

@@ -29,11 +29,20 @@
  * @brief Commands supported by the info module
  */
 enum Command {
-    CMD_UNKNOWN,  /**< Unknown or unsupported command   */
-    CMD_HELP,     /**< Display help information         */
-    CMD_VERSION,  /**< Display libdpm-core and module versions */
-    CMD_SYSTEM,   /**< Display system information       */
-    CMD_CONFIG,   /**< Display configuration information */
+    /** Unknown or unsupported command. */
+    CMD_UNKNOWN,
+
+    /** Display help information. */
+    CMD_HELP,
+
+    /** Display library and module versions. */
+    CMD_VERSION,
+
+    /** Display system information. */
+    CMD_SYSTEM,
+
+    /** Display configuration information. */
+    CMD_CONFIG,
 };
 
 /**