Draft: Define and document Hlt2 stream configuration

A proposal for how to define a Hlt2 stream configuration in a robust and simple way, complete with documentation.

Since RTA/DPA liaisons need to know how to interact with the stream configuration, I also added a first attempt at broad instructions for RTA/DPA liaisons.

TODO:

• Add comments in code where appropriate,
• Make local tests/ensure no regression locally,
• Ask for RTA/DPA liaison and RTA coordination review on instructions; ensure responsibility for keeping those docs up-to-date is clear,
• Extend to sprucing productions?
• Ensure no other Hlt2 stream configurations defined, or at least done so in appropriate place.
• Let RTA WP4 coordinators know about lines move/special mention in docs.
• Add commissioning lines to global all_lines?

I recommend this should be merged after the June-2023-data MRs are merged, as a few of them are modifying the stream configuration and it will be easier for me to keep track of that and modify/fix conflicts here where appropriate.

Closes #600.