Release version 1.0.0; keep generated documentation source-derived

The project version moves to 1.0.0, which the library soname, the
reported version, the test expectations, and the generated document
filename all derive from. SOVERSION now matches the major version.

The generated reference is built from the source and its documentation
comments alone. Prose from docs/ was briefly part of the input and is
removed: development notes are written and read on their own, and a
generator that reproduces them adds nothing. The reference opens on a
front page declared in the public header, followed by the module
contract.

One target builds it. Both PDF and HTML are produced by default, the
finished documents land in the build tree under docs/pdf and docs/html,
and the generators work in docs/tmp.

The internal namespace is dpm_core, matching the artifact name as
closely as a C++ identifier permits.

include/dpm/core.h

@@ -320,6 +320,21 @@ DPM_API const char* dpm_last_error(dpm_ctx* ctx);
 /* Module contract (implemented by modules, called by libdpm-core)    */
 /* ------------------------------------------------------------------ */
 
+/**
+ * @mainpage libdpm-core.so — API reference
+ *
+ * This is the generated reference for libdpm-core.so, extracted from
+ * the source. It covers the public C API in <dpm/core.h>, the library's
+ * internals, and the module contract.
+ *
+ * Start at core.h for the API a consumer or a module calls, and at
+ * @ref module_contract for what a module implements.
+ *
+ * The project's prose documentation — overview, design, build, module
+ * authoring, and consumer guide — lives in the docs/ directory of the
+ * repository and is read there.
+ */
+
 /**
  * @page module_contract The module contract
  *