Extend troubleshooting docs for Qt 6 and GitLab CI/CD

[NMertsch]

These are my learnings from #550. Feel free to close, ignore, or modify this PR in any way you like.

Changes to page "Troubleshooting", section "GitHub Actions":

• Change heading "GitHub Actions" to "GitHub Actions, Azure pipelines, Travis-CI and GitLab CI/CD"
• Add reference to Segfault on Linux with PyQt6 #550
• Add remark on xcb-cursor0 dependency, added in Qt 6.5
• Add remark to install libgl1 libegl1 libdbus-1-3 (not sure when they are required)
• Add example config for GitLab CI/CD

[nicoddemus]

Thanks!