Add ability to re-use part of documentation in file.

[marsfan]

Doing this will allow for us to define a complextype at only one point in a RST file, and then reference it every time it is used throughout the XSD file. This will greatly reduce the size of documentation files and reduce mistakes.

Example:

Currently we have this in docs/FS2019/XML/light.rst:

.. element:: turnLightLeft

      Definition of left turn signal paramters.

      :attrib intensity: Brightness of the light source
      :attrib node: i3d node reference to the light source.

   .. element:: turnLightRight

      Defintion of right turn signal parameters.

      :attrib intensity: Brightness of the light source
      :attrib node: i3d node reference to the light source.

With re-useable chunks of text, we could simplify this in a way like this:

.. element:: turnLightLeft

      Definition of left turn signal paramters.

      .. useChunk:: lightAttribs

   .. element:: turnLightRight

      Defintion of right turn signal parameters.

      .. useChunk:: lightAttribs

   .. defineChunk:: lightAttribs
      
      :attrib intensity: Brightness of the light source
      :attrib node: i3d node reference to the light source.

The .. defineChunk directive would not add anything to the page, but would essentially be a way to define a variable that is used every time .. useChunk:: is used with the same string following it.

I have no clue how to implement this though, so for now, copy-paste it is.