We should use relative links for images in docusaurus documents

Currently, the links in .md documents that refer to other documents or images are all absolute, e.g.

![eSSIF-Lab Architecture](https://essif-lab.pages.grnet.gr/essif-lab/images/eSSIF-Lab-Architecture.png)

However in the README.md it says: "Images must be put inside static and referred to with their relative urls."