[UNOMI-638] - Updates documentation in preparation for Unomi 2

[Fgerthoffert]

PR Title format:

[UNOMI-638] Update documentation in preparation for Unomi 2

Please add a meaningful description for your change here

• [x] What's new section
• [x] Updated quick start guide
• [x] GraphQL feature flag
• [ ] WIP...

---

Please following this checklist to help us incorporate your contribution quickly and easily:

• [x] Make sure there is a JIRA issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a JIRA issue. Your pull request should address just this issue, without pulling in other changes.
• [x] Format the pull request title like [UNOMI-XXX] - Title of the pull request
• [n/a] Provide integration tests for your changes, especially if you are changing the behavior of existing code or adding significant new parts of code.
• [n/a] Write a pull request description that is detailed enough to understand what the pull request does, how, and why. Copy the description to the related JIRA issue
• [n/a] Run mvn clean install -P integration-tests to make sure basic checks pass. A more thorough check will be performed on your pull request automatically.

To make clear that you license your contribution under the Apache License Version 2.0, January 2004 you have to acknowledge this by using the following check-box.

• [x] I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004