Added separation between instruments and between MRs.

This expects every instrument to have a {instrument}.yml file, instead of a general myst.yml. The GitLab CI/CD has been updated to build a page for every instrument in the path for that instrument, so that the FXE page would be in https://documentation-analysis-experimental-techniques-d-b26e8206e05b38.pages.xfel.eu/fxe/ and the SQS page at https://documentation-analysis-experimental-techniques-d-b26e8206e05b38.pages.xfel.eu/sqs/.

With that, we can simply change the fxe.yml file, so that content for FXE is in there, and the sqs.yml file, so that content for SQS is in there. This allows for shared content between instruments. For now, only FXE and SQS are available, but it is easy to extend for other instruments, copying one of those YAML files and changing the list of the table of contents, as required.

Additionally, the artifact name has been changed, so that is unique only within a MR and the main branch, and pages are added with a page_prefix, which allows for different subdirectories in each case.