Make building C++ API docs optional

If Doxygen isn't found then the C++ API reference page won't be included in the built docs.

docs/api/cpp_api_reference.rst

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+*****************
+C++ API Reference
+*****************
+
+.. contents::
+  :local:
+
+String Encoding
+===============
+
+* All output strings are encoded in UTF-8.
+* Input strings are expected to be encoded in UTF-8.
+
+Errors
+======
+
+All errors encountered are thrown as exceptions that inherit from
+``std::exception``.
+
+Constants
+=========
+
+.. doxygenvariable:: loot::LIBLOOT_VERSION_MAJOR
+
+.. doxygenvariable:: loot::LIBLOOT_VERSION_MINOR
+
+.. doxygenvariable:: loot::LIBLOOT_VERSION_PATCH
+
+Enumerations
+============
+
+.. doxygenenum:: loot::EdgeType
+
+.. doxygenenum:: loot::GameType
+
+.. doxygenenum:: loot::LogLevel
+
+.. doxygenenum:: loot::MessageType
+
+Functions
+=========
+
+.. doxygenfunction:: loot::SetLoggingCallback
+
+.. doxygenfunction:: loot::SetLogLevel
+
+.. doxygenfunction:: loot::IsCompatible
+
+.. doxygenfunction:: loot::CreateGameHandle
+
+.. doxygenfunction:: loot::GetLiblootVersion
+
+.. doxygenfunction:: loot::GetLiblootRevision
+
+.. doxygenfunction:: loot::SelectMessageContent
+
+Interfaces
+==========
+
+.. doxygenclass:: loot::DatabaseInterface
+   :members:
+
+.. doxygenclass:: loot::GameInterface
+   :members:
+
+.. doxygenclass:: loot::PluginInterface
+   :members:
+
+Classes
+=======
+
+.. doxygenclass:: loot::Filename
+   :members:
+
+.. doxygenclass:: loot::File
+   :members:
+
+.. doxygenclass:: loot::Group
+   :members:
+
+.. doxygenclass:: loot::Location
+   :members:
+
+.. doxygenclass:: loot::MessageContent
+   :members:
+
+.. doxygenclass:: loot::Message
+   :members:
+
+.. doxygenclass:: loot::PluginCleaningData
+   :members:
+
+.. doxygenclass:: loot::PluginMetadata
+   :members:
+
+.. doxygenclass:: loot::Tag
+   :members:
+
+.. doxygenclass:: loot::Vertex
+   :members:
+
+Exceptions
+==========
+
+.. doxygenclass:: loot::CyclicInteractionError
+   :members:
+
+.. doxygenclass:: loot::PluginNotLoadedError
+   :members:
+
+.. doxygenclass:: loot::UndefinedGroupError
+   :members: