[wip] Added inline documentation feature in xeus-cpp docs

docs/source/index.rst

@@ -39,6 +39,7 @@ The Xeus-Cpp is a Jupyter kernel for the C++ programming language
    UsingXeus-Cpp
    tutorials
    magics
+   inline_help
    dev-build-options
    debug
    FAQ

docs/source/inline_help.rst

@@ -0,0 +1,65 @@
+Inline documentation
+--------------------
+
+The standard library
+====================
+
+The ``xeus-cpp`` kernel allows users to access help on functions and classes
+of the standard library.
+
+In a code cell, typing ``?std::vector`` will simply display the help page on
+vector from the cppreference_ website.
+
+.. image:: vector_help.png
+
+Enabling the quick-help feature for third-party libraries
+=========================================================
+
+The quick help feature can be enabled for other libraries. To do so, a doxygen
+tag file for your library must be placed under the ``xeus-cpp`` "data"
+directory of the installation prefix, namely
+
+.. code::
+
+   PREFIX/share/xeus-cpp/tagfiles
+
+For ``xeus-cpp`` to be able to make use of that information, a JSON
+configuration file must be placed under the ``xeus-cpp`` `configuration`
+directory of the installation prefix, namely
+
+.. code::
+
+   PREFIX/etc/xeus-cpp/tags.d
+
+.. note::
+
+   For more information on how to generate tag files for a doxygen
+   documentation, check the `relevant section`_ of the doxygen documentation.
+
+The format for the JSON configuration file is the following
+
+.. code:: json
+
+   {
+       "url": "Base URL for the documentation",
+       "tagfile": "Name of the doxygen tagfile"
+   }
+
+For example the JSON configuration file for the documentation of the standard
+library is
+
+.. code:: json
+
+    {
+        "url": "https://en.cppreference.com/w/",
+        "tagfile": "cppreference-doxygen-web.tag.xml"
+    }
+
+.. note::
+
+   We recommend that you only use the ``https`` protocol for the URL. Indeed,
+   when the notebook is served over ``https``, content from unsecure sources
+   will not be rendered.
+
+.. _cppreference: https://en.cppreference.com
+.. _`relevant section`: https://www.doxygen.nl/manual/external.html