Issue 400d: move documentation of enumerators to the end of sections

[jkotan]

It resolves rendering problems (#400) of the latest (master branch) documentation like

/home/jkotan/sources/h5cpp-build/doc/source/api_reference/namespace_datatype.rst:75: WARNING: Duplicate C++ declaration, also defined at api_reference/namespace_datatype:11.
Declaration is '.. cpp:class:: hdf5::datatype::Array : public hdf5::datatype::Datatype'.
/home/jkotan/sources/h5cpp-build/doc/source/api_reference/namespace_datatype.rst:87: WARNING: Duplicate C++ declaration, also defined at api_reference/namespace_datatype:11.
Declaration is '.. cpp:class:: hdf5::datatype::Compound : public hdf5::datatype::Datatype'.
/home/jkotan/sources/h5cpp-build/doc/source/api_reference/namespace_datatype.rst:93: WARNING: Duplicate C++ declaration, also defined at api_reference/namespace_datatype:11.
Declaration is '.. cpp:class:: hdf5::datatype::Float : public hdf5::datatype::Datatype'.
/home/jkotan/sources/h5cpp-build/doc/source/api_reference/namespace_datatype.rst:99: WARNING: Duplicate C++ declaration, also defined at api_reference/namespace_datatype:11.
Declaration is '.. cpp:class:: hdf5::datatype::Integer : public hdf5::datatype::Datatype'.
/home/jkotan/sources/h5cpp-build/doc/source/api_reference/namespace_datatype.rst:105: WARNING: Duplicate C++ declaration, also defined at api_reference/namespace_datatype:11.
Declaration is '.. cpp:class:: hdf5::datatype::String : public hdf5::datatype::Datatype'.
/home/jkotan/sources/h5cpp-build/doc/source/api_reference/namespace_node.rst:54: WARNING: Duplicate C++ declaration, also defined at api_reference/namespace_node:11.
Declaration is '.. cpp:class:: hdf5::node::Group : public hdf5::node::Node'.
/home/jkotan/sources/h5cpp-build/doc/source/api_reference/namespace_node.rst:90: WARNING: Duplicate C++ declaration, also defined at api_reference/namespace_node:11.
Declaration is '.. cpp:class:: hdf5::node::Dataset : public hdf5::node::Node'.
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] users_guide/using                                                                                                                                                
/home/jkotan/sources/h5cpp-build/doc/source/users_guide/dataspace_selections.rst:20: WARNING: cpp:class targets a enumerator (hdf5::dataspace::Points).
/home/jkotan/sources/h5cpp-build/doc/source/users_guide/files.rst:182: WARNING: cpp:class targets a enumerator (hdf5::node::Group).
/home/jkotan/sources/h5cpp-build/doc/source/users_guide/files.rst:187: WARNING: cpp:class targets a enumerator (hdf5::node::Group).
/home/jkotan/sources/h5cpp-build/doc/source/users_guide/overview.rst:114: WARNING: cpp:class targets a enumerator (hdf5::node::Dataset).
/home/jkotan/sources/h5cpp-build/doc/source/users_guide/overview.rst:116: WARNING: cpp:class targets a enumerator (hdf5::node::Group).
/home/jkotan/sources/h5cpp-build/doc/source/users_guide/overview.rst:118: WARNING: cpp:class targets a enumerator (hdf5::node::Datatype).
/home/jkotan/sources/h5cpp-build/doc/source/users_guide/overview.rst:122: WARNING: cpp:class targets a enumerator (hdf5::node::Datatype).

I helps to breathe to get the correct links by

• moving documentation of enumerators to the end of the corresponding sections
• adding the section subtitles with :cpp:enum: and enumerator names
• changing :cpp:class: for hdf5::node::Datatype to :any: (hdf5::node::Datatype is not added to h5cpp yet so it cannot be found )

After merging the PR there is no more warnings in the documentation built of the latest version.