Draft: Docs additions: Explain streaming and RTA side of RTA/DPA liaison

Ross Hunterrequested to merge
rjhunter-rta-liaison-docs into master

Taking the still-useful docs additions from !2536 (closed), which is now otherwise obsolete.

TODO:

  • Read through to check if the information is still valid.
  • Check/address comments made on !2536 (closed) here
Edited by Ross Hunter

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