Fix Rebel Engine API documentation not displaying images

[madmiraal]

Currently, the Rebel Engine API documentation doesn't display images. Instead of images, there are links to images stored on GitHub; specifically https://raw.githubusercontent.com.

This PR enables images to be displayed in the Rebel Engine API documentation and embeds the images currently linked to on GitHub using the local copies stored on https://docs.rebeltoolbox.com.

For example, this is the Rebel Engine API Color class documentation:

Before	After

Image files still need to be added independently to the RebelDocumentation repository, but they can now be referenced and embedded in the Rebel Engine API documentation using an [img] tag. The image tag follows the BBCode [img] tag format and includes the options supported by the DocUtils Image format used by Sphinx. Spinx is used to create the Rebel Toolbox documentation: https://docs.rebeltoolbox.com/. The [img] tag options included are alt height width scale align and target. In addition it includes a caption option to add the Sphinx figure caption below the image, and it supports the BBCode shorthand for specifying the image size. For example, to embed the image shown above: [img caption="Color constants cheatsheet"]/img/color_constants.png[/img]

Includes a complete refactor of make_rst.py (which has been renamed to rst_from_xml.py) used by RebelDocumentation to convert the RebelEngine API XML files to the Rebel Documentation API ReST files. The ReST files are used to create the Rebel Engine API documentation html files on https://docs.rebeltoolbox.com/. The refactor makes the script easier to understand and maintain; including adding new features such as enabling images in the API documentation.

[DanielaOrtner]

What an improvement! Thanks for the good work. It's always best to have the images inline rather than having to go to another page.
Thanks :)