Improve rendering on GitBook

[ssteinbuss]

The IDS Reference testbed is rendered via Gitbook to https://docs.internationaldataspaces.org/ids-reference-testbed/

The configuration should be updated to:

• [ ] use the right Start Page (README.md)
• [ ] Update the TOC and navigation bar

[jfernandezsqs]

Thanks for the update and the proposed improvement in the IDS-testbed. I have reviewed the proposed changes and I think it should be included at the SUMMARY.md file the Testsuite folder.