Save external GitHub images locally and update references in versioned docs

[Svanvith]

Currently, two documentation files reference images that are hosted externally on GitHub:

versioned_docs/version-4.0/_static/env-vars/graph_readme.md
versioned_docs/version-4.0/_static/env-vars/notifications_readme.md

To make the documentation more stable and independent of external sources, these images should be stored locally in the repository.

To-DO:

Download the images currently referenced in the two files above
Save them in an appropriate local directory (e.g. within _static/env-vars/ or a dedicated image folder)
Update the image links in both .md files to point to the locally stored versions