Docs: A few small fixes to setup of development environment

What does this MR do?

• Fixes a broken link to GDK docs
• Add required config key "gitlab-server"
• Avoid config errors on startup
• Remove no longer used domain-config-source config attribute

TODO

• [-] I added the Changelog trailer (e.g. Changelog: feature) to the commits that need to be included in the changelog
• [-] I added unit tests or they are not required
• [-] I added acceptance tests or they are not required
• [-] I added documentation (or it's not required)
• [-] I followed code review guidelines
• [-] I followed Go Style guidelines