The `code` directive highlights all sources as GDScript. Other languages are
highlighted incorrectly at the moment, even if `.. code:: [language]` is
specified.
It does, however, work with the `code-block` directive. The reason seems to be
that this directive is Sphinx specific.
Also created a new `Editor development` chapter/section to encompass
C++ editor development side specifically which is not necessarily
related to pure core development, generic types and patterns.
