Review and update Ref docs

[jhkrug]

### Tasks
- [ ] System Requirements - PR #388
- [ ] Customization points
- [ ] API
- [ ] OIDC authentication
- [ ] Auth
- [ ] Cert manager
- [ ] Configurations
- [ ] Git configuration
- [ ] Application manifests
- [ ] Namespaces
- [ ] Services
- [ ] CLI Settings
- [ ] Supported Applications