Updates to GitKit-Codespaces/materials README.md

[braughtg]

The README.md for working with text / slide images in materials/ should be updated to:

• Link to the VSCode plugin for draw.io that allows images to be edited within in VSCode.
  • Check if the VSCode extension runs within the Codespace.
• Change the naming convention for images to use *.drawio.svg to be compatible with the VSCode plugin.
  • Check that with this naming convention images:
    • Are displayed by GitHub
    • Can be opened in draw.io not in VSCode
    • Can be embedded in the text.

[braughtg]

This will also add the draw.io extension to the repo so that it is recommended when the reop is opened.